RRoad BikesvsGGravel Bikes

You absolutely can, and loads of people do exactly that. A gravel bike makes a brilliant only bike because it handles everything from smooth tarmac to towpaths. You'll lose maybe 1-3 km/h compared to a pure road bike at the same effort, but honestly most riders won't notice unless they're racing.

Most road frames max out at 28-32mm because they weren't designed for anything chunkier. You'll need to check clearance at the fork crown and rear brake bridge. If you want 35mm or wider, you're really looking at a gravel or adventure bike from the start rather than trying to convert what you've got.

We'd lean towards gravel bikes for newcomers. Those wider tyres are much more forgiving on dodgy surfaces and give you confidence when you wobble (and you will wobble at first). Road bikes teach you to read the road quickly, but they're less tolerant of mistakes. It really comes down to where you'll actually ride.

Around 1-3 km/h slower at the same effort, which sounds significant until you realise that's the difference between averaging 25 and 27 km/h. On proper British roads with potholes everywhere, a gravel bike might actually be quicker because you can plough through rather than dancing around the craters.