Flag burning is not against the law per se. Of course, the flag you burn must be yours to burn (in the sense that you own the actual piece of cloth or have the owner's permission)!

Burning it in public in a way likely to inflame racial hatred or to cause fear, alarm or distress will contravene the Public Order statutes, but this is not something specific to the British flag. It is just one way of commiting a public order offence.

As for the sentiments: pretty much the same as in the US, except more diluted (us Brits are less passionate about our flag. The English flag, however, is a different matter, and is capable of inciting partiotic passion amongst our proud soccer hooligans) It would probably be frowned upon, unless you had a "good reason" (e.g. opposing Iraq war).

We just wasted a load of legislative time over here banning the hunting of foxes with hounds. I would think that a law concerning flag burning would be considered just as illiberal and as much a waste of time.