Adding my two cents, honestly, the best move for a budget is just picking up the Canon RF 50mm f/1.8 STM. It is super cheap and works perfectly on your R7. Since you have a crop sensor, it acts more like an 80mm lens, which is actually ideal for portraits anyway. Just grab the official Canon Mount Adapter EF-EOS R if you find a cheap used EF version, but the RF one is definitely the way to go.

Honestly, don't sweat it. The R7 is a beast of a camera and you really dont need to drop a grand to get sharp portraits. Since you are on a crop sensor, the field of view changes, so you want something that gives you that nice creamy background separation without breaking the bank. My top recommendation for you is the Canon RF 50mm f/1.8 STM. It is basically the standard for budget portraits. It is super compact, very sharp once you stop it down just a tiny bit, and usually goes for well under 200 quid. The aperture of 1.8 works well in lower light too, which helps for indoor wedding prep shots. If you want a bit more reach, look for the Canon RF 85mm f/2 Macro IS STM. It is slightly pricier, pushing that 300 pound limit, but the compression on portraits at 85mm is much more flattering for faces than a 50mm lens. You get a nice bokeh effect that makes your subjects really pop. I have used both extensively and honestly have no complaints for the price. Just grab one of those and spend your extra cash on a decent memory card like the SanDisk Extreme Pro 128GB SDXC UHS-I V30. Practise your framing before the wedding and you will be totally fine. You got this.

Building on the earlier suggestion, the 50mm is fine, but if you want real reliability for wedding work, you should really look into the Sigma 56mm f/1.4 DC DN Contemporary. It is built like a tank and the glass quality is just stunning for the price. I have been using their stuff for years and honestly never had a single focus miss. It is rock solid and works well under almost any lighting condition, which is a huge plus if you are dealing with indoor reception venues that have garbage lighting. Speaking of venues, did I ever tell you about that time I tried to film a wedding in an old barn? The floorboards were so creaky that every time I walked to get a better angle, it sounded like I was stepping on a bag of potato chips. I ended up having to crawl around on my knees for half the ceremony just to stay quiet. My knees were killing me by the time we hit the cake cutting. Anyway lol, sorry kinda went off topic there.

Ugh, honestly, stay away from the cheap adapters. I bought a generic one once and had constant focus issues, which was a nightmare for portraits. Are you planning to stay with the kit lens for now, or are you looking for a prime lens upgrade? If you want real quality, the Canon RF 50mm f/1.8 STM is basically the gold standard for budget portrait shooters, tho it can feel a bit tight on the R7.