HI I ordered a Free UK sim from Giff Gaff in 2018 early 2019 when in Singapore took 10 days to be delivered (free shipping(, in fact I ordered 2 at the time they seem to ship everywhere.
I activated one in Singapore and the other in Malaysia, one said no signal but I still received SMS’s. I also managed to receive my OTPs SMS in Myanmar and Vietnam.
I am not from the UK but I use it for Banks/Broker (plus other accounts) but mainly for SMS OTP.
I had a bank query once why I had a UK number with my home number detailed on bank account. I just said I travelled often with work used for EU clients, nothing since.
I add £20 by bank card every 180 days to keep it live, their customer service are excellent
When you sign up thru the link you get £5 credit so should get you going for the first 6 months, hope useful.
Ps: I also ordered an International tourist sim from HK previously which was valid for 60 days, but was able to extend topping up with my bank card have a look on amazon and ebay. BUT read tbe small print to see if refillable. Frustratingly lost my phone otherwise probably still have it but was costing about $10 month to keep active