Welcome to the muddy side 
If you just want to give it a go, you can race on nearly any bike in the local races in the UK - if you have a hardtail mtb, that’d be fine for a first race.
If not, it depends on your aspirations - do you expect to race a full season, and make this a longer-term part of cycling? If so, I’d buy a new bike. If you’re not sure, or you’ll likely only do a few races now and then for fun, a second hand bike is absolutely fine. Cantis are worse than discs when it’s wet, but absolutely fine for the odd race, so if you can get a good bike cheap, do that.
Otherwise, bikes are a bit more expensive at the moment, but there should be plenty in your budget. I did my first races on a Planet x London Road, or they have a proper cross bike too. Also some of the other UK brands have cross bikes, the problem at the moment is often whether they have any stock.
Regarding size, I’d go for the same as your road bike, or one size smaller (maybe look at the geometry and discuss with the shop, if you can’t test ride it). Some people have their saddle a little bit lower, and maybe you wouldn’t run an extra long stem to help with steering, but apart from that, I think the setup is pretty similar.
The far more important thing is mud clearance! You want to be able to run knobbly tyres and have still lots of space for mud. UCI limit is 33mm, but in many local leagues you can ride wider - very wide is not so good when its muddy, but 35 is still fine. If you can, go tubeless (or tubs, but maybe not for just giving it a go) - often you want to run as low a pressure as in any way possible. But if you can only get normal clinchers, don’t worry too much about it.
Once the cross bug bites you, you’ll want a second bike anyway, so just get one and get started…