How to cover a bicycle left outside for extended periods
Don’t worry about it. A bit of rain or dirt won’t harm your bike.

Regularly lube and clean your chain (even if it’s not yet rusty) and everything will be fine.

Salt can be quite nasty but unless you ride close to the coast it won’t be an issue in the UK.

If you want to save yourself some trouble, make sure you grease the contact area of parts such as the seat post or pedal threads before installing them so that they don't rust in place and you are still able to remove them at some point in the future. The bearings of some parts (e.g. pedals, headsets) can be re-greased which could be a good idea to do every year or so.