I'm not sure how open DCT is today to such things, Malc, but back in the 80s, after jealously and zealously protecting their copyrights throughout their history (I seem to recall some poor confectioner being besieged with DCT lawyers' letters seconds after opting to call his latest chewy bar a Nibbler, and daring to use cartoon mice on the wrapper), they opened up the floodgates to the merchandising of their characters, and suddenly I found myself drawing fewer weekly pages in favour of masses of one-off illustrations of every well-known Beano and Dandy character for packaging and products, ranging from yoghurt cartons to boxer shorts (I still have a few pairs of self-drawn drawers in my bedroom drawers). They were open to all-comers, and suddenly making a renewed fortune from their stock.
Initially, DCT insisted that all artwork was created by their 'own' artists (i.e. - mostly me), but gradually, the better drawn stuff by the approaching companies was allowable, as the demand grew and grew (and I buggered off to work elsewhere). The Harrop stuff was always beautifully detailed and crafted with meticulous care. They have an excellent reputation for quality, and I can't believe for one moment that they would take on anyone's characters without it being completely above board.
Whether these sculptor friends of yours would be entertained by DCT or not, Malc, would largely depend on the quality of their products I guess, but there would certainly be no harm in asking. It's been a long time since I had any dealings with the DCT merchandising chaps, but I'm sure they'd give any decent approach fair consideration. That said, I'm not so sure they would be the best bet for your friends - I think they opened the floodgates at just the right time back in the 80's. Tides have subsided fairly dramatically since then.