The 45p per mile (for the first 10,000 business miles - 25p per mile thereafter) is to cover all running costs of your vehicle, so you cannot claim any costs of the car separately.
Using petrol costing £1.25 per litre and a car having an MPG of around 35, fuel cost per mile is 16p per mile. The extra 29p per mile covers your maintenance and insurance costs.
You should, however, keep a log of your business mileage, just in case HMRC were to ever ask. PPs have provided ways of doing that.
Costs you can claim are those that have been incurred "wholly" and "exclusively" for business use.
So, pay a subscription to a professional membership organisation? Work related, so tax deductible. Pay for your internet at home? Not wholly a business cost, so not tax deductible.
Buying your own uniform or work related equipment, like specialist shoes, etc? Work related, so tax deductible.
Just think to yourself "would I have paid for this if it wasn't for my work?" If the answer is no, chances are you can claim for it. If the answer is yes, then it's not entirely a work related cost.
The old TV and radio advert from HMRC said "tax doesn't have to be taxing", but it certainly is! I used to shout at that advert every time it came on!
There are many accountants who will let their clients claim dodgy expenses and unfortunately those are the types of accountants that make life difficult for the rest of us.
The person claiming 6% of their car has put the car through as a business vehicle. They should then have a private use adjustment to it, to restrict how much is claimed if it is a mixed use vehicle.
For example, you buy a £10,000 car, your tax calculation shows a reduction in profits of £600 for the 6%, but if you spend 50% of the mileage on personal use, this is then tapered back to £300. If it's a "company" car then you can claim the car tax, insurance, maintenance costs back, but again the private use adjustment would be required to those costs too. You then can't claim 45p per mile in mileage, you'd have to use the fuel only rates, which vary depending on the size of your engine and the fuel used.
The quotes you've had of £400-600 do seem reasonable to be honest. Years ago you'd have found somewhere that would do it for £150 + VAT, but the cost of living crisis has affected businesses too. Some firms would allow you to pay over the year on a monthly standing order, which might help?
Do your own this year and speak to a local firm about next year?
Payments on account only need to be made if your final tax bill for this year comes out at over £1,000. If you've only earned £8,000 in self employment, once you've calculated your "profits" you might find it's less than that anyway.
Ultimately by 31st January you would ordinarily pay your tax liability for 2023/24 and then 50% of that amount again as a payment on account towards 2024/25.
You'll then pay a second 24/25 POA in July, then any balancing liability by 31/01/2026.
Bear in mind that that 2026 payment would then also include a POA for the next year too and if you're doing a whole year of self employment, will be higher.
Well done for putting money aside each month though - many of our clients don't do that and then have a panicked scramble each year.
If you can't afford the final payment due, speak to HMRC ASAP, or use the online service - https://www.gov.uk/difficulties-paying-hmrc/pay-in-instalments to request more time to pay.