Japan still has several thousand tonnes of spent fuel in store and reprocessing it would reduce the storage requirements if nothing else. I figure they will restart most of their nuclear fleet over the next few years, the cost of importing more LNG to make up for the cheap nuclear electricity they're not generating is starting to impact the country's financial bottom line.
Britain's current nuclear fleet of 12 reactors generates about 8GW or roughly 20% of our electricity needs running flat out between refuelling stoppages. One small reactor at Wylfa, the last operating Magnox unit anywhere I believe, currently producing about 420MW to the grid may be shut down next year after 40-odd years of operation when the last fresh fuel elements are expended. Plans have been announced to build two EPR1400s with a total grid capacity of 3.2GW at Hinkley Point, an old Magnox reactor site and more recently a two-reactor generating plant with a similar capacity but based on boiling-water reactors has been announced for Wylfa, both to come on-line in about ten years or so when the other reactors currently operating, nearly all AGRs, start to reach end-of-life after 40-50 years of operation. The four Hinkley Point and Wylfa new-builds by themselves will replace about 80% of the capacity currently provided by the ten AGRs, and the new designs could operate until the end of the century. There's also an 1100MW PWR at Sizewell with a projected operating life out beyond 2050.
Reactors have been getting bigger over the past few decades -- some future designs are in the 1800MWe class -- but the extra size makes them more expensive to build. Fewer units are needed however for the same capacity in a grid and operating costs are generally lower per MW of capacity (reduced staffing, longer operation periods between refuelling, smaller footprints etc.) It's a problem (to get back to the original subject) that the NuScale and mPower small modular reactors face, loss of economy of scale. Yes, they can be built in a factory but most large components for an EPR1400 are factory-built anyway; moving one large reactor vessel or ten small ones to a site is a wash in terms of cost and disruption and any nuclear build will involve a lot of ground works, concrete and rebar which can't be done on a production line indoors.