I would suggest you have 2 options here:
1. A, not too expensive, suit. You can get something wearable for £200 from Debenhams. Slaters or M&S. It won’t be the height of sartorial excellence but it will do a job. Go for navy blue and team it with a pair of good shoes (black or brown, not square toed), a matching belt and a lightly patterned shirt (light blue or pink will do the job, nothing too loud) with no tie.
2. The dangerous world of “smart casual”. blue or “khaki” Chino’s or smart grey trousers, team it with a light blue or pink Oxford shirt, a navy or tweed jacket and a pair of brown shoes (ideally brogues).
PErsonally, I’d go with a suit as you can wear it again for weddings/funerals/christenings/etc. Plus, you’ll struggle to get a suit wrong, smart casual is a whole minefield of sartorial sins in waiting.