We were about to buy a house, the paperwork from the seller has turned into a mess and we have pulled out.

 

I don't want to remit the funds back out of Thailand, the baht has taken a dive since then and I don't want to pay tax bringing it back in next year if we need to.

 

I figure the best move is to mirror our offshore investment,s but in Thaland for the time being

 

Which bank or finance service provides access to the LSE and is straightforward. It will be in my wife's name if that makes it easier.

Banks don't provide access to stock exchanges - brokers do.

I'd suggest Phillips POEMS - used to dealing with foreigners.  https://www.poems.in.th/

Even though banks often own brokerages it's (usually? never?) not possible to open brokerage accounts through your local bank branch.  You'd need to deal directly with the brokerage.

An exception is for UHNW individuals.  For example, if you've got 50 million baht in your bank accounts, Krung Sri will provide access to foreign stock exchanges.