Most pubs that I go in are licensed until at least 1am on a Saturday night anyway. (They might not actually stay open that long but they're allowed to if they want to).
The Licensing Act 2003 basically states that a pub should be granted a licence for whatever hours are asked for, unless there's a good reason why not. That's led to some wide discrepancies. In large areas of North Norfolk nearly all pubs asked for 24 hour licences and, in the absence of valid reasons against such proposals, the council granted such licences. (So the Royal Wedding 'relaxation' can't make any difference at all in most of North Norfolk)
Councils are meant to consider each licence application individually, based upon the merits of that application (and the validity of any objections). It's unlawful for them to operate 'block' policies. Despite that, the London Borough of Havering has a policy of not allowing any licences after 11.30pm, except in the centre of Romford. So the residents of Havering (outside of central Romford) might benefit from the temporary change to the rules.
Here in Suffolk, most pubs are licensed until at least midnight during the week (although many will actually close at 11pm, or even earlier) and until at least 1am (often 2am) at weekends. (Again, only a few pubs actually stay open until those times but they won't need any 'Royal Wedding extension' in order to do so).