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-run business based in the Midlands, established in 1997 to supply and install non-mains drainage systems across the UK.

By 2004, growing customer demand led us to offer a full range of drainage solutions from a single source - serving mainly the self-build and renovation market with all underground drainage needs.

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.