The 'official' first day of Spring is 21 March. It's hard to tell if the climactic conditions that normally define Spring are getting earlier - I think it's more a case that Winter is much milder, Summer is less consistently warm, and the seasons are starting to blend into each other, with less of a recognisable divide. As Woody Allen said, "If you don't like the British weather, hang around for ten minutes!"
A reasonably reliable guide is Hawthorn blossom. If its out its spring, if not it aint. Unlike Blackthorn, hawthorn (or may blossom...confusing as it normally flowers in early april) is dependant upon both temperature and light cues for blooming. It is rare to get a frost after it bllooms, hence that old cliche "cast neer a clout till may is out".
Another indicator is that easter eggs appear in the shops and the weather on telly has a background of daffodils despite the knee deep snow outside.