I know I may get shunned for saying this but strawberries are a close option. I have bought some random things on Amazon though and I’m pretty sure you can find guava paste on there. Found it. Idk if Walmart is in the UK but they also can ship it internationally.

https://www.amazon.com/Goya-Guava-Paste-21-Ounce/dp/B00060N4U2