If you’re going to buy anything buy trousers, boots and impact shorts (the basic 661 bike ones with no rigid plastic are okay for boarding good and cheaper than boarding crash shorts) You’ll need you own gloves. Winter bike gloves would do the job. A BMX helmet is fine for the dome – no wind chill.
Jacket isn’t really necessary at the dome – I find a thick hoody with a long sleeve T and a base layer underneath is fine.
MK will give lend a board that’ll be fine to learn on and you can try different sizes easily. If you develop a bit more the Snowboard Asylum will have a variety of demo boards and good advice.
SCUK membership should get you decent discounts on time at the snowdome.
Not buying a board right now and not getting too carried away going to MK every week will be a good start to saving up for your first trip to the mountains 🙂
See the other recent snowboard threads if you absolutely must buy a board. You’ll find an astonishing lack of consensus. April is a good time to buy – but I’d wait until next April – and buy new.