Assuming at some time in the future the UK does actually leave the EU, does that mean that websites hosted within the UK no longer have to comply with GDPR?
Even after that, the government has stated that even if not signed up to the GDPR, it will insert the laws into new British legislation; they recognise that people need to be protected from both tracking and unwanted advertising --- plus of course, even the loonier brexiteers know that some people will want to do business with Europeans, and if so will have to comply with European laws, especially concerning the GDPR.
However already the GDPR is 'incorporated' in the British Data Protection Act (2018): a good summary is this:
By now it is impossible to imagine what our panic-stricken and demented Commons plans for Brexit Day, but it seems an even chance they will have to beg for an extension to Article 50 to sort out the issues they had time to address over the last 2 years; and a tiny but hopeful chance they may just withdraw it completely. Either would mean less to worry about.