The lemon tree in my conservatory has started to flower, earliest it's done so as usually it's around Feb before I get any flowers. Should I start watering and feeding it now, it could probably do with potting up too.
I've got a few different citrus fruits which I inherited from a local orangery. If they need re-potting I would do that first. You could water sparingly when that's done. I wouldn't do much else until you see signs of vegetative growth (we could yet be hit by a cold snap). My fennel is growing vigorously. but I have a feeling it's tempting fate. :)