According to the terms at the very top of the Seed Exchange Forum, GardenWeb can only be used to exchange seeds within one's own country. There are legal and ethical implications, regarding sending seeds to and from other countries, to consider as well.

It used to be strictly against the rules to ever ask for free seeds. Since Houzz took over, I don't know if it's still a rule, but doing so is considered by many as improper, at least. It's always possible to buy a few seeds to "prime the pump".

I'm not sure if there are many regular members in this forum from the UK. If there are, I haven't noticed. While you are most welcome here in GardenWeb, you might want to look into UK-based sites for seed exchange to make things easier. Also, there is a Gardening in the United Kingdom Forum here on GardenWeb. You might want to try that forum.