You can get either from Amazon UK or other online stores probably for a cheaper price than in retail stores...
There are a lot of stores that stock something like this, whether through their online stores or in actual stores on the high street. Stores like Game and GameStation (if that still exists, I haven't seen one in London) specialise but other places like Argos, and even supermarkets like Tesco can stock them.
Sample prices from Argos's website suggest around £80 for a basic console and £135 for a 2DS XL. That is without specifying a colour.
https://www.argos.co.uk/browse/technology/video-games-and-consoles/nintendo-2ds-and-2ds-xl/nintendo-2ds-and-2ds-xl-consoles/c:30008/Same really applies for the games although you can probably get cheaper on exchanged second hand games at stores like CEX.
https://uk.webuy.com/I have seen copies of those games in CEX before, and I think I picked up a spare copy of X a while back from one of them. They also have consoles I think. The only negative of those is that if she got one second hand and there was a problem with it, you wouldn't be able to exchange it.
I haven't bought anything since my 3ds years ago but I don't think it matters whether in London or not, most of the stores and availability are nationwide and would deliver to a UK address anywhere.
Some stores also sell refurbished consoles via ebay at a discount.
Amazon UK suggests a price of around £32.99 for a new copy of X, though there are some other options which vary.
Does any of that help you?
Most UK and European consoles also don't now come with chargers. I honestly don't know if the US charger would be compatible - I would say yes but I remember there are different voltages between the UK and some other places, so worth checking out first.