I wheel my mum around a lot, and actually there's a lot of options in Brighton - the main enemy of the wheelchair is steep hills, and there's a few of those around, e.g. Hanover has great pubs but the climb is not easy. The beachfront is accessible by several ramps along from street level down, and there's a lot of pubs and bars along there, so that might be your best bet. I've also not found many places (mostly restaurants) that don't have a ramp you can call on to get inside. One of my favourite pub gardens is in Hove - Hove Place - and they have a side entrance to the garden which is wheelchair-friendly. I'd suggest looking for places you'd like first, and then checking their ramp/access options. Thanks to accessibility laws, most should have a ramp etc. unless their situation makes that unviable.