From Thomas Cook Airline's website:
You are allowed one piece of hand baggage only which must not exceed 5 kg in weight or 43 cm x 28 cm x 23 cm in size
So TC's restrictions are a lot tighter (about half) of BAA's. Your holdall must be smaller than BAA's requirements (most "holdalls" are bigger than them), but I don't know how strict Thomas Cook are with their own requirements (since I have never flown them). Maybe someone else can tell you about their recent experiences with them?
Usually airlines are concerned more about the weight of bags than the size. As long its roughly around the BAA guidelines then I wouldnt worry, Ive seen people taking huge bags as hand luggage and they always seem to get away with it!
We have occasionally had someone look at our bags going out, but its never a problem as long as you don't go totally overboard you should be ok. Coming back they never seem to bother