Firstly 5G uses completely different frequencies than 2/3/4G. 4G is still being upgraded across the UK, and in-fact O2 now has secured more spectrum for 4G which it has been using to upgrade existing masts over the last year or so and will continue to do so into 2020.
Secondly, something that is reliant on the government however (so may be effected by the election) is a network sharing plan for rural areas, so that all networks gain much greater signal coverage without having to build new masts, this could improve things greatly itself.
5G is also much much faster with far better throughput, so by putting it in well built up busy areas it will reduce the load on the struggling 4G network, so even those still using 4G will actually get better data speeds and reliability in those areas.
5G isn't a miracle fix, in-fact on the most part it won't change things to much, but the difference it does make in the areas it is or will be implemented is a positive one.