Ok, I've plugged your numbers into a compound growth calculator and an excel spreadsheet. If you don't spend any of your lump sum, you are likely to be £30k better off in 12 years if you take the bigger lump sum than if you take the smaller lump sum. I've copied the amounts at the bottom of the post. My calculations assume 7% growth on the investment after inflation.
Option 1 lower lump sum after 12 years you will have had £170,814 net of inflation from investing the lump sum and taking the higher annual income.
Option 2 higher lump sum after 12 years you will have had £199,500 net of inflation from taking the higher lump sum and lower annual income.
I haven't taken tax into account, as that is dependent on your other income streams - eg if you are at stage pension age, you will get your state pension, if you have other private pensions you could have an income from those, or if this is for early retirement and you will continue working full or part time, then all these affect tax
But, it's not just the benefit of the £30k you need to think about.
If you have that money in your ISA, it's yours, none of it will be taxed and you get the control of that money, instead of the government. Personally, I really dislike the gvt trying to control money that's mine! You can spend it, support your kids at uni, or contributing to a house deposit, or the holiday of a lifetime, or several holidays of a lifetime! In 12 years time you could be too old or too ill to spend that much money, or you might die. Having the money up front gives you options.
You'll need to put £20k in by 5th April then £20k on 6th April, and any extra will either be taxed on the dividends or you can use your husband's ISA allowance for a year then move it into your own name in 2027. Or hold it in a non-isa high interest savings account for a year at 4% and you won't pay tax on the interest at all as you'll be below the tax on on savings threshold.
If you are taxed on the dividends, then the tax is 8.5% vs 20% income tax you'll pay on the annual income. If you haven't reached state pension age, then there will also be NI of another 8% to pay on top, that you don't pay on dividends.
(Assuming you have other income to take you above the £12500 basic rate tax threshold, and if you are taking your state pension, then that will be tax free but your TP will be taxed)
Regarding and IFA, please don't use one. If £30-50k is all you have in S&S then their costs, plus the cost of the ISA platform, plus the fund charges will make a huge dent in your capital. You really need to have over £100k-£200k to make and IFA worthwhile. If you use an IFA you will lose £12k over the 12 years which is HUGE and any decent IFA should tell you it's not worth your while. And if they don't tell you that, then you shouldn't use them anyway!
To be honest, it's not a major deal either way, as £30k over 12 years is not set in stone, it's only £2500 pa, and it depends what you do with it. You might blow it all in the first year or two! But at least you'll have had the choice to do that.
Option 1
Lump sum
£27,000
7% after inflation, compounded
£33,810
Total after 12 years
£60,810
Annual income
£9,167 per year for 12 years = £110,004
Value over 12 years net of inflation
£170,814
Option 2
Lump sum
£49,320
7% after inflation, compounded
£61,760
Total after 12 years
£111,090
Annual income
£7,367 per year for 12 years = £88,404
Value over 12 years net of inflation
£199494
Hope that helps
Jopo