Following various interventions from Ofcom over the years, it has become a lot easier to switch broadband providers. In most cases the provider you are switching to will handle the whole switching process for you once you've placed an order on their website.
So, my advice would be to wait until you're out of contract with your existing provider, then call them to haggle for the best deal they can give you. If you're not happy with what they offer, use a comparison site like https://broadbandinternetuk.com to find a better deal, then place an order & switch provider.