What changes after 28 days is that you cannot apply for an interim possession order. You would start with rules of civil procedure, part 55.21, which says:
(1) An application for an IPO may be made where the following
conditions are satisfied –
(a) the only claim made is a possession claim against trespassers for
the recovery of premises;
(b) the claimant –
(i) has an immediate right to possession of the premises; and
(ii) has had such a right throughout the period of alleged unlawful
(c) the claim is made within 28 days of the date on which the claimant
first knew, or ought reasonably to have known, that the defendant (or
any of the defendants), was in occupation.
(2) An application for an IPO may not be made against a defendant who
entered or remained on the premises with the consent of a person who,
at the time consent was given, had an immediate right to possession of
Accordingly, you can apply for an IPO under UK law if you do so subject to these conditions, one of which is the 28 day time limit. Fill in this form, sent it to the local court, they'll confirm and send you some documents which you give to the squatter (48 hour timeframe), etc. then they can be arrested if they don't go away (for at least a year). Otherwise, you file a "claim for possession". This page will start you on your quest to discover what that involves – money, court hearing, and time.