The whole rationale behind the carriers is that they must provide a significant contribution to airpower in theatre. That means a big deck. We have not managed "quite well" with CVS and certainly not in terms of strike. The best we could do in the Gulf in 2001 was put up two four-ship SHAR/GR7 combo's once a day, twice at a stretch, which still needed significant USN support to get over the beach. That is not to denigrate in any way what those lads did, but in essence it was not a military capability, but rather a political statement from UK.
The "smaller" ships have been extensively looked at and just can't provide the sortie rate required for long enough and more importantly run the risk of becoming too small for future ops well before their mid-life, as CVS did. To get funding for a programme these days you have to be able to justify that capability - buying a ship less capable than the military requirement and saying we'll "make do" or "manage", will get that programme sh1tcanned sharpish.
In any case, going to a smaller ship now would put the programme back another five years at least and would not result in a massive cost-saving (you're talking tens of £M tops, not the hundreds you'd need to make a real dent in the cost). I am personally struggling to understand how these simple (and they are) ships cost £1.5Bn each. There is around 25000 te of steel in the design as is, at say £700 /te and a competent shipyard should be able to get the manhours per tonne down to 150 hrs /te. At a labour rate of £20/hr plus 100% overhead, I make that £167M per ship (12% of total). I know outfit materials and rates are much more expensive, but not by that extent - particularly for a ship with bog-all in the way of weapons.
In any case, as pointed out by Brains, aircraft programmes are massively more expensive - Typhoon is upwards of £30Bn. I'd remember that before talking about "unaffordable" ships.