"Enough ground forces to ensure that UK soil is not taken over" would require a couple more armoured divisions (we have *one*), and several more mechanised divisions (again, we have just one). Defending the UK mainland from ground forces hasn't been practical since WW2.
Arguably we have enough "Navy to keep the shipping lanes open", but which shipping lanes do we care about? We can't defend the Atlantic, and we barely have enough capability to defend tankers coming from the Middle East, without the extra cost of defending overseas territories (e.g. The Falklands), peacekeeping/interdiction operations or humanitarian aid.
About the only thing we can defend is our skies, and even then the odds are against us if an enemy gets that close.
Pushing the burden onto the UN/NATO sounds good but is a textbook example of the Tragedy of the Commons - everybody assumes they can use it, nobody supports it, so the system collapses.
We have, basically, three options:
1. Abandon our overseas territories and any notion of being a military power, disband the army except for a token peacekeeping force, disband virtually all of the surface fleet, and hope that nobody ever does anything to threaten us or our economic interests, as our only options would then be a) cry or b) nuclear war.
2. Buy the cheapest possible hardware to save money in the short term - the Abbey Wood approach. Of course, since most of this matériel is foreign, we'll lose our manufacturing base, and then the options are either pay whatever the US charges (and their lack of cost-control is legendary) or go to option 1 (do not pass Go, do not collect £200m).
3. Support the Defence Industrial Strategy and buy enough homegrown tech to avoid skills fade. This probably needs a small budget increase, and definitely relies on the MoD and industry to cooperate.
Paris, because she knows what it takes to keep squaddies happy.
AC for obvious reasons.