I had about 4 cheap Cats that cost around £60 but she was failing the emission test every time even last year the reading was 0.29 CO2 when the limit was 0.30 so just made it after putting in a bottle of Dipotene into the petrol.Make sure you get the correct one for the car, the design changed around 2000 to accommodate the pre cat sensor so the cat will have longer with the bend in post cat tube fitted. I should think your F will be of the latter design, but you never know.
If you are planning on keeping the car I wouldn’t go for the cheap Cats on eBay, they tend to last as long as the next MoT. Given the cost of the OEM part is around £350 and that would likely last another 20 years. I have no experience of www.cats2u.co.uk but they are selling a type approved cat for £250.