I agree that it does sound like a micro-burst although these are not normally associated with the UK climate.
I believe I suffered one in Central America about twenty years ago, flying an RAF Puma helicopter with 14 POB. As we entered a rain shower (not at all unusual out there in season), the aircraft was basically propelled downwards against full power. There was no turbulence as I remember, just a powerful downwards surge as the rain intensity suddenly increased markedly and we lost visual references.
As full power and a flare from 120 kts to 70kts (Vy) had little effect in getting the VSI needle off the bottom stop, for a few seconds I had the sudden thought that both engines had flamed out (which of course they had not or I would have had no rotor rpm worth talking about with max. collective pitch applied).
I carried out a steep turn through 180 degrees and a few seconds later, as we came out of the rain, the aircraft suddenly climbed like a cork out of a champagne bottle. The event began at about 1500 ft and we recovered to the climb at about 200 ft above the jungle. I had put the gear down as I was convinced we were going in! I went straight back to the departure helipad a chastened man and we stayed there overnight. No-one argued, especially the crewman.
I too could not understand what had happened and no-one I spoke to had experienced anything like it. I later discovered an American paper on micro-bursts. Basically, mass ascent of air over a wide area can sometimes result in an extremely violent downdraught in the vicinity of Cu-Nim clouds. This is usually associated with an area of heavy rain.
Perhaps you experienced a similar thing, albeit rare for UK due to the lower i.e. non-tropical temperatures.
Was the air unusually unstable for the UK?