Originally Posted by DfT

Advice for Disabled Travellers from Abroad The blue badge introduced a standard design of disabled persons' parking card across all European Union (EU) Member States. This enables disabled visitors from within the EU to enjoy the parking concessions provided in the UK by displaying the badge issued under their own national scheme.
For visitors from countries outside of the EU where no formal arrangements currently exist, UK parking enforcement officials may be prepared to recognise your disabled motorists' badges, although that will be entirely at their own discretion. We suggest that you bring your badge with you and check with the local police in the areas you intend to visit.
Details of the concessions UK blue badge holders are entitled to are contained in our explanatory booklet on the Scheme.

Originally Posted by Tourism for All

For overseas visitors, there are no reciprocal arrangements between Britain and the USA, however, most places, with the exception of Central London, will honour the US sticker, providing the car is safely parked. Rules related to disabled Badges are different in relation to the country you are coming from; therefore we strictly suggest you to check with the authority at your country about relative restrictions or validity.