AppleCare+ generally provides cover for compatible accessories purchased in the same transaction as the iPad. As such, an Apple Keyboard and Apple Pencil purchased with the iPad will be covered by the AppleCare+ plan.
Accessories purchased in another transaction - whether directly from Apple or an Apple Authorised Retailer - will not be covered by AppleCare+.
As such, whether you purchase your Magic Keyboard from Currys or not is moot - as it will only have benefit of the retailers warranty and Apple’s limited one-year warranty. As you are assumed to be in the UK, you may have additional statutory rights.