Help with drawdown strategy

So roughly 3% income wanted from 300K pension plus 250K ISA?

Just mulling the simplest plan, which I know isn't objectively the best, but let's see how it does.

1. Crystallise everything, 225K goes into drawdown and 75K tax free lump sum.
2. Assume 250K ISA, 225K drawdown fund and unused portion of 75K in dealing account are all invested in the same way
3. Withdraw enough from pension each year to use up personal allowance
4. It takes about three years to use up almost all the 75K, 20K goes into ISA each year plus a few K taken to get income up to to target. The tax on dividends from 75K will be virtually nothing, I calculate £20 for the first year, assuming 2K dividend allowance and no CGT.
5. Once the lump sum is used up, supplement pension from ISA.

Don't let natural income be a constraint, it's very artificial and means you won't be looking at the issues that matter. But if you must use that as your withdrawal algorithm, then realise that selling 100 shares in one account and buying 100 in another means no change in the overall number of shares, you've effectively just shifted 100 shares from one account to the other.

In terms of maintaining allocations, just have a spreadsheet where you add up your shares across all accounts, in the early years you will be buying in one account while selling in another, you can make buy and selling choices so you are constantly moving towards you target allocation.

It's not too bad once you get used to it. The getting used to it bit can take a while, though!

There are a range of options and different ways to do things to cater to the different situations that people are in, so each can do whatever is best for their own situation.