Having had a couple of disasters over the last couple of years (I say disaster in a philosophical way as anyone who takes U.K. holidays knows weather is an occupational hazard!), I've done a bit of layman's analysis of how the weather has behaved over the last few years, and there is a definite trend that May tends to produce a dependable amount of dry and sunny weather compared to other months, April also to some extent but less reliably so. There has been a definite trend over the last few years for July to be dodgy, which does fit with the general trend (even in the well-regarded 2018 summer there was a tidemark, albeit that year it came later than usual). Another observation is that once things return to unsettled grey skies mixed with wind and rain it seems to have had a habit of remaining like it for the rest of the summer.
Ultimately we perhaps got lucky over April and May that as well as fine weather it wasn't too hot, which could also have been problematic with people queuing in the open-air. A solution will need to be found to this, as it's going to lead to frayed tempers - I get the sense there's already quite a bit of pent-up anger and frustration floating around, and queuing outside in the wind and rain isn't going to help that.
One other semi-related thing to watch will be if many people decide to do "staycations" this year - bad summer weather could well result in a couple of scenarios, some deciding to cancel at the last minute, or others deciding never again - neither of which would ultimately be good for UK businesses.