If I buy a simcard (for data) in London, will I be able to use it in Belgium.
Yes, mobile services have no roaming charges and work across the EU since June of 2017.
Yes. Vodafone SIM and plan I picked up in London last year worked fine in Switzerland. Planning to do the same this year. I got one of their big value bundle plans. Good for 30 days. If you’re trip is longer you may need to figure out a way to top up (renew the plan) or get a Belgian SIM once the first one expires.
Maybe. I've read reports that some providers like Giffgaff might require you to use the SIM in the UK for a period first before they enable EU roaming. This is apparently to try to weed out tourists and limit EU roaming to residents. But - try it and report back!
Thanks all!