The UK does have nuclear power. As far as I recall it's about 20%, gas is around 40% and the rest is hydro and wind. But before anyone argues about whether to increase nuclear or not, realise that the UK is one of the best places in the world for offshore and onshore wind energy potential. There's therefore an argument against increasing nuclear production, ignoring all of the other issues for and against, simply because there is so much free wind currently unexploited. Possibly some extra nuclear power will still be needed to cover the base load on the rarest of unwindy days, but even that argument must take into account the potential for interconnections to Scandinavia and mainland Europe. The nuclear vs wind debate is probably different in other countries with less wind.
We did, in 1956 and continue to invest.
Current budgeted future spending is £129B+ for cleanup, and £20B+ for Hinkley C. That will grow if more new sites are built.
In the UK we're always about 15% nuclear power[1]. Removing the huge proportion of gas should be next on the list, ideally with more zero carbon and nuclear power.