It was Mr Roy when I was first watching Basil, but there are a few old videos knocking about with Mr Derek (nowhere near enough of original Basil Brush material in circulation, though).
Basil was superb. When a compilation DVD of original Basil was released in about 2000, I promptly purchased it to enjoy his laugh and humour again (of course I knew it worked on two levels), only to realise that a lot of my own mannerisms and expressions were directly stolen from Basil all those years ago.
Two videos with Mr Derek: