Tina, the Nutricentre sell a liquid magnesium chloride though it's not listed on their website. The brand is "E-lyte" and it's £9 something for a bottle pre any discounts. I've only just started using it so dont know how long it will last (maybe a month or so).
I know iherb do a few liq mag chlorides which are much cheaper but obviously the overall order has to be worth the standard $20 P&P to the UK.