@neddie: Younger people aren't interested in organising something like voting days or weeks in advance. They just want to turn up to the polling station on the day and do the job in 5 mins.
And you know this...how, exactly? Or are you just stereotyping all "younger" people (age range undefined) as feckless slaves to instant gratification?
No-one is making postal voting 'mandatory' (your word) for people who have booked holidays already; in case you hand't noticed, we don't have mandatory voting in the UK. So people with holidays already booked have two options: apply for a postal vote, or not bother voting. The former option being one which people complaining about the timing of the election seem to either ignore, or be ignorant of.
(And a postal vote is free, so it's not adding any cost to the likely planet-destroying flyaway holidays that many of these people will have booked.)
I'm with Morningsider on this: pity the poor souls likely to be inundated with postal voting applications. And with that in mind I can actually foresee a potential barrier to voting if the volume of applications means that they won't be able to process them all in a timely manner, and some people end up not getting their postal voting forms before they go on their holidays.
Out of curiosity, I did a quick check on Wikipedia and it does seem that the latest 'summer' election in the 20th and 21st centuries prior to this one was the one called by Harold Wilson for June 18th 1970 - the one he lost to Edward Heath. Elections in July/August were quite common back in the 19th century, though summer holidays were probably less of an issue in the days before universal suffrage and statutory annual leave entitlements. And apparently up until WWI elections lasted several weeks.
So it does look as if the timing of this upcoming election is somewhat clumsy, though whether the date was chosen deliberately as form of "jerrymandering" (as Rees-Mogg incorrectly referred to the introduction of photo ID) I doubt, simply because it would suggest a degree of sophisticated insight and planning which seems to be beyond the capabilities of the current government.