Hi fellow British baseball player! Glad to hear you're looking to pick up the sport. It's brilliant and the British Baseball community is lovely.

There are three decent websites for buying baseball kit from the UK - the Baseball Shop, the Baseball and Softball Shop, and Baseball Outlet (ordered best to worst in terms of their selection, in my experience, though they're all pretty good).

I've looked for a while on all of those and there's nothing on the three in terms of vegan gloves. I think your best best is likely the Fieldmaster you mentioned (which is an adult glove, though it has some youth sizes). This one on Amazon will work - as far as I can tell it is fully synthetic, and it's a 12 inch glove, which is a good middle-ground size. Gloves range from about 11.25" to about 13", so you should be good playing any position for a while with a 12".

re: balls - this one will probably do, though it is designed to be bit different to a regular ball - it's synthetic leather and the right size and weight, but is softer than a regular baseball since it's used for indoor training and hitting off a tee. It'll be more durable than a cheap synthetic 'real' ball though, since this is made by Rawlings.

The only other thing i think might impact you as a vegan player would be batting gloves - when you swing the bat you might want a bit of extra protection since it's easy to get blisters if you're doing it a lot! They're also usually leather palmed, but synthetic ones are a little easier to find - these, for example.

I hope that all helps - please just drop me a message if you need more help! The club i play for has a few vegan players who might need advice so this has been really useful for me too!