One option is to pay by card if you have one associated with your Santander account. Obviously this is a lot more expensive (depending on the card it will set you back up to 1,000 baht per 1,000£ transferred) but if you need money quickly it can be handy.
Another option is to open a Wise account in GBP (a pretty straightforward process) and transfer money to it in (either by wire transfer or card) in advance. You can use the associated Wise Mastercard for withdrawals anywhere including Thailand (the Wise fees are low but of course there is the 220 THB fee), make payments such as on Lazada etc., and send money to your Thai bank account in a few seconds when needed.