Re: I hope they succeed ... but! Economics!
I never found out whether that was a stunt
No not a stunt, just an artifact from slinging a huge load of wind and solar assets at the grid, and having operating and payment arrangements such that these assets become "must run", and indeed even get paid if there's insufficient demand or grid constraints.
Today's a minimal coal day (see for yourself, search on the terms templar and gridwatch), but as I write, the reality is that nuclear is running at full chat around 8 GW, the Dutch and French interconnectors are importing the better part of 2 GW, Drax is burning the forests of Louisiana to make 2 GW, and the biggy is gas turbines are humming away to the tune of 16.5 GW. Wind is producing a miserable 1.39 GW, merely serving to push coal off the grid at the moment, and thus require "Capacity Market" subsidies to keep it available.
Going back to your point about SMR, I've had some limited professional involvement, and I think your view on costs is correct - that by the time you've got something buildable, the cost per GW will be astronomical. However, this won't stop the mad fools of the British government. With their obsession about saving the planet by cutting CO2 emissions, they hope to decarbonise not just electricity and transport, but heating. And what they (wrongly) believe is a possible solution, is to have cities linked to large and astronomically expensive district heating systems, using the surplus heat from small, local nuclear reactors - these SMR. You might think that's utterly, utterly mad, but this is laid out in the government policy documents (eg Chart 18, p46, The Future of Heating, DECC, 2013). Obviously there's no heat demand around Trawsfynydd, Hinkley Point or Sizewell, so the bureaucrats plan to take the nuclear reactors to where there is heat demand.
The cost of solar PV built so far in the UK averages about £150 MWh across the portfolio. Hinkley has a contract for £92 MWh, but at 2012 prices, so uplifted by CPI we're already at £104/MWh, and by the time it is operational (I guess) in 2032 it'll also be up to about £150/MWh. These SMR will need the same sort of price (even if simpler than an EPR, they lack the economies of scale). Which means the average commodity element of electricity bills will roughly triple by 2030. Add to that all the money being frittered on energy storage and network reinforcement for all the crappy renewables, and the plans for another seven nuclear reactors, of about four different designs and people in the UK won't be able to afford to have electricity. And they won;t be able to afford heat, either, because the cost of district heating is about £10k per connected property.
You couldn't make a worse mess of energy policy if you actually set out to do things wrong intentionally.