As long as you realise that LD40 is a thermal insulation foam, not buoyancy foam as such, so can't guarantee it will not absorb water.
As to your requirement to local sources in/near Bristol, DIY superstores only do builders foam and it is unlikely that ironmongers will have anything that specialised. I did look on Force 4 Chandlers, not too far from you, but could not see it listed. Chandlers are full size boat specialists so more likely to stock buoyancy foam, which should not force out bulges.
If you have sanded through the hull skin you will need to repair and reinforce that first, not rely on pumping foam in the hole.