While it is internally consistent to withdraw from NATO if the UK chooses not to defend other NATO countries, as that is the key mechanism holding the alliance together, the UK would not be capable of defending itself without NATO (and primarily US) assistance in the event of an attack from a non-NATO country, and therefore it would be unwise to do so. Furthermore, the existence of Article 5 has a deterrent effect, reducing the likelihood of such attacks in the first place. The costs of remaining in NATO, i.e. defending other NATO countries with low military spending, are enormously outweighed by the benefits.