I'm guessing it's going to be £350. The Switch has a custom Nvidia Tegra processor inside and it's rumoured to be around Xbox One levels of power, which for a handheld is pretty incredible. That plus the modular peripherals is why I think it will be that high vs the 3DS launch price, but this is console spec we're talking about.
I think Nintendo are going all out to get a lot of early adopters in the hope that third parties will stay on board after the PS4 Pro hits its stride and the Xbox Scorpio is released, which the latter especially will leave it in the dust specs-wise. So Nintendo have to offer an attractive platform for developers to release their games on, either ports of the PS4 and Xbox One versions of their games (for as long as "no console left behind" is maintained by Sony and Microsoft, which I'm guessing will be two years at most) or third-party exclusives. Otherwise it will end up like the WiiU and who knows if Nintendo will ever make another home console. Personally I think this will be another Gamecube; a quality console that does well enough for Nintendo to continue, but not a generation-winner by any means.