We are indeed testing 2 accounts at the moment, we will keep you posted on this!
The only way of moving stock from a GIA to an ISA is to sell it and buy it back, as unfortunately you cannot transfer stock between the 2 accounts in this direction.
What we are able to do is help you buy back into your ISA as quickly as possible, so the price you get is as close as possible. So, if you place your sales in your GIA, once they have executed, let us know and we will move the money to your ISA before the trade has settled. This means you can buy the stock back in your ISA on the same day
Regarding doing this for free, you are right that you would need the money to be in your ISA first, so you couldn’t queue both sides at the same time as the buys would fail. To get both sides with no dealing commission you could queue the sale on day 1 and queue the buy after it had traded and we had moved the cash so it would likely execute on day 2.