I got a pair of Bosch Blue 18v 5 years ago, drill and impact driver, still very happy with them. Got a DeWalt recently for my daughter, equally good. Buying process was simple, stick to main brands like DeWalt, Bosch, Makita and see what Screwfix or Toolstation, locally Anglia Tool Centre, have deals on. I remember when I got the Bosch a really good deal was the twin pack, could have paid more and got the single pack drill only. For my amateur use battery size isn't important. I have 3 x 1.5 ah which means I can have one on each tool and one on charge, rarely use away from home so a bigger battery won't add much usefulness. I can see why it helps if you are a professional and using it all day every day. The bosch struggles a bit with the bigger hole saws but they are old, I blame blunt cutters rather than the drill.
Your choice might be limited by stock availability.