Exactly how monumental an undertaking this would be may be gauged from the fact that the current 17% of total electricity produced by nuclear is generated from 18 reactors, which are housed in 10 separate power stations. On this basis, to substitute for the 80% of electricity currently produced from coal and gas, using nuclear, would require building around 100 new reactors, and that is on top of the new ones that will be required in any case, to replace all but one of the existing reactors, which will come to the end of their working lifetimes by the year 2023. This clearly is a colossal undertaking which does not solve the major issues of "security of supply" or CO2 emissions in any significant degree. We will still need to import oil and gas from politically maverick regions, mainly Russia and the Middle east, and is the uranium fuel required for nuclear to be found on our doorstep? Hardly. Most of it comes over from Canada.
What about renewables? It is thought that in the longer run (say, by 2050) around 40% of the U.K.'s electricity might be provided using wind/wave/hydroelectric/ solar power. A significant proportion of this would be produced by "microgeneration" devices, rather than a large scale "grid", though any excess electricity generated beyond the local demands of each "micro" community, could be fed into the central network. This still only addresses "electricity" as a final fuel, and the question of providing the greater bulk of "energy" persists.
In simple economic terms, on the level of an individual or a country, the degree of security depends on the gap between income and expenditure. More can be earned or less spent. As far as the U.K.'s energy earnings are concerned, the limit is in sight. We cannot realistically "earn" more fuel, and we may well have to endure a pay-cut. It is thus a matter of economy, and of economising. That we spend the precious resources of oil and gas only where it is essential to do so. This will involve schemes of energy efficiency, for example the "40% House" being researched by Dr Brenda Boardman's group in the Environmental Change Institute at Oxford University and the Passivhaus concept.
Such advances in building design could make huge savings in energy use for "space heating" across both the domestic and commercial sectors (each of which accounts for around 30% of the national total energy demand). Transport, which uses another 26%, mainly in the form of oil, is another area where savings could be made, both through more efficient combustion engines (or fuel cells, if the costs can ever be made realistic), and simply by eliminating all unnecessary use of cars (especially the military style "road wagons" - 4x4's, SUV's, depending on which side of the Atlantic you are - that have more to do with symbolising status than any practical transportation issue) .
To a reasonable mind it all seems straightforward, but I suspect there are too many people making too much money to allow any attention more than lip-service to be paid, until it is too late and there is no longer any choice.