1) The only 'cancellable' Typhoons are the 90 or so in Tranche 3 (the jets that might have CFTs and all the bells and whistles that would make them really useful A-G aircraft).
2) You will only save any serious money if you can get the other partners to cancel their T3 aircraft, too, otherwise it's penalty payments. ("It would be cheaper to take delivery and then dismantle them with a JCB" someone once said).
3) You're looking at saving at most £3.8 Bn, if you can get the above ducks in a row, and save every single penny from Tranche 3. You'd save that at a single stroke by canning the carriers alone, and you'd save MUCH more if you also canned JSF, for which we still don't have a definitive price, a realistic ISD, or proper reassurances on ITAR, and which would represent an additional aircraft type, with an additional logistics chain.
If you want kit that's useable today, then JSF ain't it. Perhaps we need a shedload of Gripens or F-16Es to augment the Typhoons.....
4) If you can't get HNS, the op is probably politically unsustainable anyway. (And you need HNS for the AAR/ISTAR, etc.)