Opening a CREST personal member account is done through the stock broker who sponsors you, rather than by dealing directly with CREST. No stock broker gives you CREST membership automatically – it’s an option you need to ask for.
There are a very limited number of UK stock brokers that currently offer CREST sponsorship. The personal CREST account comparison table on the front page of this website lists the main ones that will do so.
Some other traditional full service stock brokers may also offer this service, although they will probably be relatively expensive. The Association of Private Client Investment Managers and Stockbrokers has a searchable list of members on its website that includes a option to look for firms that offer CREST membership.