Around a century ago the army made a series of deep reforms that came out of the experience of the second Boer war. These left us with a standing army capable of forming an expeditionary force, with 'twinned' territorial units at home re-supplying and training, and with the militia and yeomanry for home defence. Arguably, without these reforms we would not have been able to slow the Germans at Mons, Paris may have fallen and the war may have been over by October 1914.
Times and needs change, and without apportioning numbers I'd suggest we need to look for the following capabilities;
An Expeditionary capacity - capable of both independent deployment and of forming part of a NATO deployment, useful both on the European plain against armour, in asymmetric conflicts, police actions, and across a variety of operational environments from desert to arctic.
Reserve / training capacity - More closely integrated TA (as is happening) plus far greater use of university cadets, bursaries, shared apprenticeships, work-service sharing etc - offering defence roles, training and experience to all members of the UK workforce 17-47. Tesco van drivers who can pilot a tank.
Militia / Yeomanry - A revival of the traditional bodies used for home defence, also deployed in civil areas for public re-assurance during periods of terrorist threat etc., independent of the Police and under the command of the Lords Lieutenant and not the MoD under an integrated home defence strategy. Service to 65 years.
The Air Force - Less independent role and more to establish air superiority where ground troops are operating, to support them with ground attack, for resupply and rapid deployment of ground troops, global deployment of unmanned aerial vehicles (UAVs) with both surveillance and offensive capacity
The Fencibles - Revival of volunteer, part-time coastguard service assisting HMRC and for inshore coastal defence, policing small boat traffic etc - Service to 65.
Not definitive and I'm definitely open to argument on everything - but clearly we need a new aim and purpose for defence rather than just nibbling at out-dated structures to achieve 'austerity' armed forces. If this can all be done for 2% of GDP, great. If it takes more, we must find it.