Bots??

Some people have the page to refresh so when it changes it lets them know.

Sell £0.01 so that the other person can go to the buy page, put in the full amount and pass the reCAPTCHA. The first person then sells the remaining amount to transfer. The buyer is already to press the buy button.

Some people get up in the middle of the night to do this so this gives international investors a chance to snaffle.

Try not to operate the two accounts yourself, may open up a T&C can of worms.

The above is the best answer to the original question.  I do this all the time and it has never let me down.  On the buying machine/account you go all the way except to click the final "Cancel/OK" confirmation.  When you click OK to sell on the selling machine/account, the amount seems to be instantaneously available, although I usually wait about one second before clicking OK on the buying machine/account just in case.

Spouses don't automatically have power of attorney over the financial affairs of their partner do they?
When my father became unable to cope with his financial and Property affairs we were advised that either my mother or myself would need to apply for a lasting power of attorney. His wife, my mother, could not do financial things without legal authority. As I was younger and more up with financial and money matters I agreed with my mother that I would apply for a lasting power of attorney over his affairs rather than her and once that had been validated I was able to do things quite legally for my father.

I have his full authority obviously, we share a laptop anyway and the second level security comes to my mo as he only has a company supplied one.

I don't want to breech some random terms and conditions.

PoA is a bit of a minefield in that different financial institutions and banks and government departments seem to have different policies on it.
With some I had to go in and produce all the documentation and register myself on the account as power of attorney and others wanted me to bring it all in every time for every transaction and some online people wanted things scanned in and sent and others didn't seem to give a hoot.
What I was getting at was that certain people have commented that they are running accounts for various family members including their children and doing a lot of trading and it seemed extremely unlikely to me that those other family members had a clue what was going on in terms of all the transactions and that the accounts had only been opened in other peoples names to grab extra allocations of new loans which through clever means would then be bought up by the account managing the ring.
Some of these online platforms are very slack in that respect in that if you know somebody's full name, address, date of birth etc. you can sign up to accounts in their names all over the place and the computer has no idea you are not them.
Clearly if it is a matter of a husband and wife who live together and share a computer and possibly have a joint bank account and discuss all their financial arrangements that is a different situation altogether.