Don't forget the 10cm cylinder has to sit really low, ideally flat on bottom of the hull in order for none to project above the waterline if you're hoping to minimise the size of the ballast tank and achieve a true scale waterline.
Modern Brit subs seem to be a cross between Russian and U.S design principles- we employ single skinned hulls with forward and aft free flood areas, like the Americans, but favour the high freeboard that Russian boats have.
If you're running the rods externally, that is going to increase the height of the cylinder by at least 6-7mm, so it's pretty likely that the cylinder will poke a bit above the waterline. With a 20cm long tank, assuming your pump can fill it to 75% capacity, that gives you about a litre of displacement. The Vanguard needs about 750-850ml to lift the top hull, sail, vanes and rudder above the surface to scale waterline. I've allowed a 10-15% fudge factor, as hand laid GRP hulls can vary a bit. So you have about 150-250ml in hand.
Lets assume your cylinder is 100cm long, 5mm projecting above the waterline will require about 150ml of extra volume in your ballast tank. Should be okay with that.
If it projects 10mm above the waterline, you will need an extra 400ml. Now you're going to be in trouble. So watch that height so things don't get out of hand.