AIBU to think I might get heavily taxed?

Yes a tax rebate won’t be immediate.

But I’d have thought it unlikely that any extra tax from temporarily losing the tax free allowance would exceed the extra income from temporarily having two jobs at once?
Even if it takes your payroll a while to sort the tax codes out once the gardening leave is finished?

Can you tell me what are these jobs where you earn £60-70K despite being unable to understand basic tax rules?

Worst case, if they did tax you on that basis for a few months, then wouldn’t you get a tax rebate after the end of the tax year when HMRC get your P60 with your actual income for the year and do whatever number crunching goes on at their end?

Redundancy payments should be tax free

HMRC will do it, though the payroll at your new job may do it first.

You will get a form called s P45 from your previous employer when they remove you from their active payroll. That will state how much you earned in that employment since April 2025 and how much tax they have deducted. You give a copy of this form to the payroll team at your new employers (make sure you keep a copy too). It may be that their software will be able to do the correction during the subsequent few months.

After the end of March 2026 you will get a form from your new employer called a P60 which will state how much they paid you and how much tax they have deducted.

If you don't want to do the maths yourself you can just plug the total earned and total deducted from those two forms into a self assessment form and if you paid too much you will be able to get a refund.

However, for the example I gave, the reason someone would be due a refund would be because the double-payment income would make a basic-rate taxpayer look incorrectly like a higher-rate taxpayer when they are not. If the actual numbers are double that then no such misidentification would happen. Both employers will correctly recognise you as a higher rate taxpayer and will correctly deduct the appropriate tax.

For rough numbers, if the two employments are both actually paid at around the £70,000 pa mark:
Your nornal monthly gross income is £5,833
Your normal deduction for income tax would be £1,286.
In the overlap month it will appear as if your gross income annual is £140,000, you will have an actual gross amount recieved of £11,666 and you will pay £3,619 of tax. This is correct. no adjustments necessary. You pay 40% of everything you earn over £50,270 in a year, no matter how many employers it comes from.

I think you can transfer it yourself and tell HMRC, with proof, that you've done it. You should then get the tax paid back.

Why don’t you get everything from your first employer paid into your pension? Problem solved.

I can't advise on the tax codes, etc. side of things but if you're worried that once any overpayment of tax is sorted you'll still have paid a lot of tax, can you sock a bunch of your income into pension to get tax relief on it?

If your total income from April 2025 to March 2026 inclusive is still under the threshold for 40% tax (£50,271) then you won't be heavily taxed in the long run, you'll just pay your standard 20% on everything over £12570 like everyone else. You might pay too much tax in the overlap month, if you get paid an amount in that month from both employments which is more than one twelfth of £50,271 - but it will be easy to claim it back, and the amount will e negligible if you are on an ordinary sort of income.

Eg if both your old and new jobs pay about £35,000pa but there's an overlap month where you get pay from both, you will appear to have an income of £70,000 for that month only. So your gross pay in that month will be £5,833 rather than the nornal £2,916 and you may have an extra £328 of income tax deducted which wasn't correctly due. However, you will still have thousands more than you do in a normal month, so no real harm done. At the end of the tax year the process for getting back the difference is not particularly problematic.

You may need to check with your current employer to see if you are ok to work for someone else while on garden leave. Most companies don't let you as it's used to protect their interests

If your current company are happy to continue paying you if you take up a new job during the notice period for which they are paying you then you will be taxed higher for the overlap period but you will actually have the two incomes. Once the notice period ends and they submit the P45 the tax will be adjusted and you will get a higher sum the following pay day and then goes back to normal so it sorts itself out .

You can also go online to your HMRC account and adjust the amount in your account for estimated income for the current employer to reflect what you will earn for them between April and leaving date too.

That's cool, in which case, HMRC will tax based on what you have earned during the year. Unlikely your tax code will change until you hit a threshold.

PAYE is calculated on a cumulative basis. This means that once you’re overlap period ends, the new employer will calculate the PAYE due on the total income for the year, deduct th PAYE paid to date, and you pay the difference. You might end up with a couple of unusual months, but you shouldn’t end up with less net pay in the overlap months than you would have with a single job.

NI is different, because it’s charged on a monthly basis, rather than cumulatively. However, you still shouldn’t end up worse off than having a single income for the overlap months

I would be more worried that your current employer will have terms and conditions that prevent you taking another job while you are on gardening leave, and will want to cease paying you if you take another job during gardening leave.

It will be like people that take large sums out of their pension or have a large bonus, I don't know how long it takes for HMRC to catch up nowadays that this is not your monthly salary for the whole year.

I don't understand this, you'll be taxed on what you're paid.

So if that is going to be 100k, then yes, you'll be taxed on earning 100k.

Or do you mean you'll be earning 2 salaries of 50k for a short period of overlap and you're worried HMRC will tax you on 100k for the whole year?

If so, they won't, if you're on PAYE it will sort itself automatically and you'll get a rebate if necessary.