One of the key aspects of DB'ing is not arguing and not having relationship talks. You got caught up in exactly that yesterday with these exchanges. It seems you're trying to use logic to convince him on what is an emotional decision. Have you read up on listening and validation? Shorter responses without arguing or trying to convince him: E.g., "I'll have to think about that" or "That won't work for me".
Are you employed? And if so how much do you make relevant to H (less, more, the same)? Why don't you have access to marital bank accounts? Why would he ask for £4,000 to cover taxes, is that a typical arrangement for you two in previous years? You already paid him £8,450 for taxes a month ago?
I completely agree w/kml on protecting your finances. Not sure about where you are, but most places in the US would consider his income marital assets, and in many places if he's spending significant money on an AP you may be entitled to recoup that money. You should consult an L and know where you stand, and how to best protect yourself.
Me:39 Ex-W:37 M:7 T: 9 S:6 D:3 BD/IHS/Confirm EA/PA: Feb '20 OM1 affair ends: May '20 W/OM2 & moves out: June-July '20 W files for D: Jul20 OM2 confirmed: 9/2020 Divorced: May '21