A fairly simple calculation will tell you how much rainwater you can harvest.
For the calculation below, the roof area is the flat area of ground covered by the roof (not the pitched area). Average rainfall figures for many UK locations are available from the Meteorological Office. Suppliers should be able to give estimates for the other figures.
Roof area (in square metres) x Annual rainfall (in mm) x System efficiency (e.g. a 'WISY' filter is 90%) x Run-off coefficient of roof (a pitched, tiled roof is 0.75, but a flat roof 0.5 or less)
= Annual collection in litres (1000 litres = 1 cubic metre, or m3)
A tank would normally be sized to store 5% of this. Water should enter the storage tank just above the bottom - so as not to disturb debris collecting there. It will be drawn from below the surface using a submersible pump - to avoid picking up pollen etc from the top. You also need to check that your roof is clean and not made of toxic metals.
A filter that is 90% efficient when clean can drop to below 30% if algal growth blocks the holes in the mesh, so they need to be in a position where they can be accessed and cleaned occasionally.
There is much more detailed advice on doing this calculation, and other aspects of rainwater harvesting, in the book Choosing Ecological Water Supply & Treatment, available from CAT.