The Yorkshire Envelope Company make envelopes. Yorkshire Envelopes were part of the the Mail Solutions Group (see below) but in October 2013 it was announced on the Pukka Pads website that they had acquired Yorkshire Envelopes. Yorkshire Envelopes is a leading manufacturer of a huge range of envelopes (bubble Envelopes, business envelopes, home envelopes, greeting card envelopes, money envelopes, etc.), however it specialises in envelopes for greeting cards of all shapes, colours and thicknesses. Yorkshire Envelopes still make their envelopes in the UK and have a factory in in Bingley, West Yorkshire. Available online on their website – http://www.yorkshireenvelopes.com
Mail Solutions own a number of envelope manufacturers in the UK and India, including BHF Envelopes, Crossways Envelopes, Empire Envelopes and the newly founded Great British Envelopes Limited (which appears to be the new trading name of Empire Envelopes). I am not sure whether any of these brand names are still used or if only the Mail Solutions branding is now used. Empire Envelopes Limited trading as (t/a) Great British Envelopes & Print do have their own website. Empire Envelopes also have their own web address (online shop) but it is branded as Mail Solutions. Mail Solutions manufacture envelopes in Britain and India. They do not give country of origin information on their websites, so you will need to ask – http://www.mailsolutions.com
If you shop around you should have no difficulty in getting British made envelopes.
For more British made envelopes and associated products, please see my article on British made greetings cards, envelopes, gift wrap and postcards and British made stationery.