This grade business means little to many of us. I recall that it involves increasing levels of competence for school children (maybe limited to the UK), but keep in mind you are engaged here with a lot of older folks from around the world, so Grade 5 means little or nothing to us. For sure, it means nothing in the selection of your new clarinet, if that is what is necessary.
For instance, I am nearly 70 and have been playing reeds for over 50 years. I taught myself clarinet. I play exclusively original jazz from the 1920's, but I can play clarinet and own several, including my main clarinet - a Bb soprano Buffet RC Prestige that I bought new at Howarth's in London in 1985.
So, regardless of "grades, etc." what you want is an in-tune wooden clarinet that responds well, has a good tone, and will remain in your hands for many years to come. I think you need to have your parents take you to a large instrument store (Howarth's comes to mind because I've been there, if they are still in business) where you can play several models and compare them to what you are currently playing. That is exactly how I did it for my Buffet.
True, you may be able to save a few pounds by buying sight-unseen off the Internet, but for the money involved, that seems a bit risky to me. Better that you play what you buy before you buy it.
As far as repairing your current clarinet, a complete 50-pound overhaul may not be necessary. Replacing a few felts and leaking pads may do the trick for now. I can't speak to repair costs in your area, but I'd bet a minor repair would be a lot less expensive than a complete overhaul. AND, 50 pounds for an overhaul seems to be way too cheap, in my opinion. DAVE