Are these maths right? Should I be maxing pension once my isa is full?

Assume you are higher rate tax payer of 40% just before retirement and have 100 pound to invest.

Scenario 1. Big pension already(flexible drawdown)
100 avc is put in tax free
25 pound tax free can be taken at retirement out of this pot
If you have 20k of income you can use flexible drawdown which will have 20 percent tax rate up to 40k/annum. Value 80%x75= 60 pound if you stay in this limit
Ie 85 value total

Alternative is to take 60 pound out after tax.
Ie 41% better off with pension when considering the avc as a marginal increase.

Scenario 2 smaller pension(annuity)
If you havent got a big enough pension pot ie before 20k and assuming forced to buy an annuity

Still get 25 pound tax free
The £75 becomes worth £3/year after tax (which sounds rubbish and is at a low) calc here is 5%x80%x75
However taking it now you pay tax and your £60 will only offer you 1 pound a year equivalent (safe 2 percent bank interest but will also be taxed but clearly you can spend the 60 so flexibility is useful.)
The annuity catches up if you live for more than 12 years(the 25 pounds tax free means only 35 to close).

Scenario 3 not at retirement
Key other advantage with the pension it's able to grow tax free and the 40% also grows and contributes because the tax comes when you are taking a pension rather than now.
So if I invest both in shares with 5% dividend for 10 years
My 60 pound income becomes 81 due to higher rate tax(assuming isa is full)
My 100 pound avc becomes worth 155. So gains are 2.6x as much in the pension.
Plus capital gains isn't an issue. Getting access to the gain may incur tax but this is likely to be at a lower rate so dropping 20% off this becomes 124 still a 53% gain.

If this is right why shouldn't I funnel all my money in(40k cap is going to be tricky to hit)?

Robin
your calculations appear to be right. Alan is right that UK share divs are taxed at 10% even in ISA / SIPP, BUT bonds, bond funds and some overseas domiciled collective investments (e.g., AIF and UEM) do NOT have tax deducted at source, so escape the 10%.
The attractions of pensions are much as your describe, but my order of priorities would be:
1. ISA, 2. SIPP, 3 VCT (for taxfree income).
But I would not be so keen on SIPP if I thought I should be obliged to take 75% of fund in form of an annuity. But, as you say, if you have over £20k of other income, you can opt for flexible drawdown.

Regardless of pension pot, the age for being 'forced' to buy an annuity has been raised (at 75 now, I think) and for anyone with a SIPP who is 65 today, I suspect the rules will be relaxed even more before the 75th birthday is reached. If I'm wrong on the age limit, let me know, someone.

I would not invest in a pension in anticipation of a new tax regime that may or may not ever be introduced. I would use ISAs to the max., and I would not 'over-invest' in a pension now if that means any of your investment will attract tax relief at anything less than 40%. After all, the tax treatment of pension income may also change (I think the same think tank advocated reducing or abolishing the tax-free lump sum).
As ever, do not make investment decisions solely or mainly on the basis of their apparent tax advantages. Alan's suggestion of letting low / no-icome growth investments build up unrealised capital gains would certainly be attractive if CGT rates were to rise above their present rates.

In a financial climate where governments are hungry for funds to stop the economy's wheels falling off (because of earlier bad management and failure to rein in over-spending by the public at large, and financial shenanigans by certain financial firms), any assets that are fixed and not accessible because of restrictive rules can become sitting ducks. So any pension fund that you are not allowed to draw on because of 'rules', or any property that by its nature cannot be uprooted and taken abroad to a more benign tax climate (see Spain for how badly this can turn out), is at risk when official thievery becomes a problem.

At least with an ISA you might stand a chance of cashing in at any time, and might then have the chance to ship it abroad out of harm's way if they haven't suddenly reintroduced exchange controls.

Many people around the world now seem to favour having money in more than one jurisdiction, preferring those that are thought less likely to confiscate what is not legally theirs to take. Precious metals in a secure warehouse in Singapore, a bank account in, say, Uruguay, tax residence in neither, all these are ideas I've come across. I wouldn't be keen to buy a house in Spain at half the present depressed price, since that makes you liable for Spanish tax on your worldwide income at whatever rate the Spanish taxman thinks he can extract from you. Who is to say that other cash-strapped countries cannot change the rules whenever it suits them, and use your hard-earned money to cover up their own past mistakes?