Get Sonic Forces for Nintendo Switch for only £19.99 (or less depending on the exchange rate) at the Mexican E-shop.
Although from Mexico, it's exactly the same as the European version with all languages (including English and Japanese) and includes all the bonus costumes. Buy before 24th of November to get the Free Shadow costume.
The game is between £27 - £35 elsewhere, so it probably wont be any cheaper than this until after Xmas.
How to buy:
1) If you don't have money on your Nintendo Account, the easiest way is to change your account region to Mexico (You can change back to UK after the purchase, but you'll lose any money on the account when changing regions).
If you do have money, then you'll have to set up a new Nintendo Account on your Switch that is set to Mexico. You'll still be able to access the purchase from your main account, but you will now have multiple accounts cluttering up your Switch.
2) Tap on the shop icon (on the Switch) and search for Sonic Forces.
3) Buy using a pay method that works on the Mexican store (most UK credit cards and Paypal do not).
Once bought you can re-download at anytime from the UK Eshop. However, if Sega release new DLC, you might have to switch back to the Mexican shop to get it, as UK DLC doesn't seem to work with the game.