Welcome to the forums, @EmmaW97 . If you only have a small flat, but have been losing your Wi-Fi signal when you move away from the router as you describe, it sounds as though the Sky routers signal is really weak.
The signal from the stock router supplied by PN isn't that great, but I can't believe it is that bad - don't know for certain, as I don't use it.
Are you going to be on Fibre with PN? - if so, there is plenty of advice on the forum if you have problems, and a number of experienced users who can help if you get stuck.