Since we are on a water meter and we get about half the national average of rain, we have been saving rainwater ever since we moved in 20 years ago. At present we have 12 barrels attached to various down spouts to collect almost every drop that falls on the roof. Sadly we can empty all of them very quickly once the rain stops.
Good idea though, rain water is better for many plants than tap.

Connect the pipe from your guttering to a barrel(s) - simples!

No disadvantages as rain-water is purer than tap.

As to lower rain-fall - the answer is to grow perennials rather than annuals as these have time to develope deeper roots down to the moisture in the sub-soil.

And mulch heavily when the soil is wet!

Well I'm still waiting for this drought - no hosepipe bans yet!

Proper rainwater harvesting systems cost thousands of pounds - even the smallest tank (1500 litres), before installation, is about £2k. Like most efficiency measures it's best done at build rather than as retrofit.
You still need mains supply water for drinking/cooking so there's very little financial saving in the UK (water companies won't give you a reduction on metered supplies because you're still, ultimately, putting water into their drainage system.
But when all's said and done - if it isn't raining what are you going to harvest?

Hi, unforunately, {or fortunately} we don't have this particular problem very often, we usually get enough rain, and if by a miracle we get a prolonged dry period, well we're not on a water meter, so just use the hosepipe, I think the last time we had a hosepipe ban was 2006, and the day after the ban came into force, it started raining, and hardly ever stopped for about 3 months, by which time it was winter, so no problem, Derek.

I had to have a new water supply last year when I bought this house, to replace the lead mains pipe. I applied for a water meter to be installed at the same time but that was too much for the water company to cope with. So I'm not on a metered supply here and my water bill is less than half what it was at my last house (nothing to do with the amount of water I use because I don't use any less now) so I don't think I'll be rushing to get a meter installed after all.