The RAFs currently has in service the following combat aircraft Nimrod, Harrier, Tornado and Typhoon, it plans to move to replace the Tornado and Harrier with the F35B, Lightening II, which it will share with the Fleet Air Arm to equip the Queen Elizabeth class of carriers of which two have been placed on order.
The B version of the Lightening is designed for STOVL operation, there are rumours that the order may be changed to the C version which is designed for CATOBAR operations and has greater range and is cheaper than the B version. This would require modification of the carriers but this was allowed for in the design. Thought that it would save the MoD some $3.4 billion, which is approximately the cost of one of the carriers.
The original design philosophy behind the Harrier was that any Warsaw pact attack would likely destroy the RAF's runways but a STOVL aircraft could operate from clearings or the remains of runways. It also allowed for VTOL operations from forward positions. The use on the invincible class of carrier was purely an after though and while useful for smaller ships seems pointless with the larger ships that are actually planned.
The Warsaw pact is no longer seen as the major enemy and NATO has not found a technologically comparable foe to replace them, definitely not one capable of destroying airfields deep within it territory.
It is possible that NATO my face a to major threat in the medium term from the middle east, if it can create a coherent force but given the trials and tribulation in Europe that prospect is at least 30 years away. At worst in the near term, the middle east provides an on going drain to western forces, using equipment obtained from outside the region. One that will have to be addressed by piece meal operations on land, sea and air and by the control of weapons provided to the area to limit the distance at which action can be taken.
If NATO has a major military threat anywhere in the world then that threat is China but while China has a growing capability it still lack the global reach to strike at the heart of Europe, any war with China will be fought mostly in the east and against submarines in the North Atlantic that pass under the north pole, neither of these cases is best addressed by the planned large STOVL carriers. It requires aircraft that have range and payload operating from either a conventional carrier or from land bases for operations in the east and with small escort carriers, hunter killer submarines and Maritime patrol aircraft to secure the Atlantic and other sea lanes.
In order to fill this requirement the RAF and Navy need F-35C and conventional carriers, plus a long range bomber. In the short term extra Nimrod aircraft could fill the gap and in the longer term a specialist bomber is needed not just for the RAF but for all European NATO forces, currently only the US possesses aircraft with this capability namely the B-52, B1 and B2 aircraft. In the past only the UK also possessed such an aircraft in the form of the V bombers.
The US is unlikely to sell B2 to anyone and it is unlikely to restart production of the B1, proposed development of the B1R concentrate on the modification of existing aircraft. Given the protracted development of the A400M and Nimrod MR4A, it isn't going to be cheap. However it is the capability which is missing from an independent European defence force.