I'm afraid your not in luck. Sales season in London is late June/July and late December/January.
Forget about those shirt makers in Jermyn Street where the the 'Easter Sale' is followed by the 'Spring Sale', followed by the 'Glorious May Sale', but on the whole this time of year is not bargain territory.
Until now, the spring has been unusually cold, so most retailers haven't made any inroads into selling their Spring/Summer collection. Some of them might be desperate and are putting out reductions already, to get some of their stock moved out of the door.