A heck of a lot more expensive! Four for £8.99, as opposed to eight for £5.50?? I vaguely recall the Flightpath set, and seem to remember they were rather good,( as most of their stuff is) PE and resin I believe. But, the price seems low for their products - you sure you've got the prices the right way around. BTW, I can't find the spare RP's I had for you. I've found the fins (which are cr*p anyway), but cant find the RPs and rails!!
I think at £18 to furnish the Beau with RPs, the same price as the kit itself, I'd stick with the kit's own AP rockets!
EDIT:- Just noticed, there's only three rails and four RP's, two of which are 'piggy backed'. You might have to buy three sets to get the required number of rails, ie, eight rails and eight RP's.