I have no experience of them, Jim, but using eight sprinklers will cause quite a power surge and I doubt that small solar units would cope. They will also empty your water butts rather quickly.

Can`t answer your question Jim sorry, just wanted to suggest you put your pic up on the main pages, your garden looks lovely and would then be seen by all, hope you don`t mind me saying...
Welcome to Goy...

What marvellous lilies! have never tried power assisted watering - I always do it the hard way. Good luck with that though.

A drip system on your in-ground shrubs and flowers would be more efficient. and would require less pressure than sprinklers. Thick mulching with coarse compost, coarse ground bark, or wood chips, will also help with water conservation.
Containers are pretty happy with drip, too, but you will need that so often that it will empty your butts beaucoup fast. With containers it is often better to save the rain water for occasional medicinal purposes--unless you get a lot more rain than has been usual for the UK, lately.
As for the lawn, I would use the tap water with sprinklers, but use it very efficiently. Try to water more deeply, but less often. If the soil is slow draining, use a sprinkeler that applies water slowly, so you lose less to runoff. Water early in the morning, to reduce losses to evaporation. Keep your soil in good shape with regular applications of compost, or compost tea, to improve water penetration and retention. Yearly aeration also helps with this. If needed, switch to a more drought tolerant variety of grass, such as Tall Fescue. In times of drought, it can be simpler to let your lawn go to sleep, since it wakes up again easily, once the rains begin again in fall--a judicious fall feeding will help its recovery.

I would use bedding plants with the lilies you can have different ones every year.

You don't say how large is your garden. Ours is 40 by 50. I have ten dwarf apples and live in a dry part of England....Essex. I make a lot of use of direct watering with two litre plastic bottles sunk near trees and plants I want to fuss over. 3 butts, watergel crystals, mulches. I let the grass go dry and it comes back as soon as it rains. We have certain ways in the house that would not suit many people, but it keeps our metered bill in check. You need a round table discussion about shower times and frequencies. How long clothes are worn before they hit the washer. Shorten the wash too and use something like Napisan, if anyone freaks out over germs from a cooler, shorter wash time. Water is humanities lifeblood and meters can make you think about how it is used.