"So you have the power sources sorted, the actual infrastructure can gradually be upgraded as required. It isn't that difficult to upgraded a substation or increase the UHV cable capacity. As long as it is gradual and not required in 5 years time. It's only a bit more difficult than getting FTTH for everyone."
Some unquantified caveats there. Having watched the progress of the build of an additonal feed from an offshore wind farm into the National Grid the progress is very slow. Infrastructure seems to be (quite sensibly) underground these days and a new link has to go through/past existing infrastructure which all takes time and planning. A lot of time seems to be taken up by archeology along the chosen route. You could of course just cover the whole country is a massive web of pylons and overhead cables.
I don't think it is fair to compare the issues of running very high power lines all over the country to running fibre down urban streets and out to country villages.