Tanks Direct

https://www.tanks-direct.co.uk/more/rainwater-harvesting.html

Above and below ground water storage tanks, we offer a full range of plastic, steel and GRP (fibreglass) water tanks, cold water tanks for the loft and large galvanised steel water tanks for agricultural use.  We also offer food-grade water containers that are suitable for potable (drinking) water, chemical storage, wastewater and general cold water storage. 

Drain Store

https://drainstore.com/rainwater-harvesters-stormwater-management-and-water-attenuation-units/

Drainstore is a family business based in the Midlands. It was formed in 1997 initially to provide a UK wide service supplying and fitting of non-mains drainage sewage systems. In 2004, we recognised that our customers were more frequently asking to be provided with all their drainage requirements from one source. Drainstore was set up as a service aimed predominantly at the self-build and renovating market to allow them to source all their underground requirements from one supplier.

Tanks for Everything

https://www.tanksforeverything.co.uk/water-tanks/domestic-rainwater-harvesting-systems-p

There are many benefits to storing rainwater in a rainwater harvesting system, the main one being the saving made on metered water costs which can see up to 50% savings. Other key benefits to harvesting rainwater include contributing to reduced flood risks and lowering the load on combined sewer overflows improving river water quality.