If you're going to be away for two weeks and have noone to trust to tend them, then i'd probably harvest (unless in a greenhouse, as previously stated). It certainly won't be as good as if you harvest when they're ready (think green banana) however it'll still get you high, just not in the same way, or for the same duration, should it get harvested when it should.
The way i see it, better a jar of prematurely cut weed, than a jar of mould. While not ready, i say harvest them. This is the UK, who knows what the weather is going to bring tomorrow, let alone over the next fortnight.