I guess it depends on whether you want a defensive force to defend the UK, or do you also want to defend the Falklands, St Helena, Bermuda, Gibraltar, etc.
Because if it's the former, then UK armed forces as they stand should be adequate. Our island nation is geographically very defencible, it would take an enormous effort to create any meaningful beachhead.
If it's the later then that involves a much higher level of logistics, maintaining a significant presence around the globe in order to react swiftly to any threat, which the UK forces are probably not able to do as effectively as would be liked.