I am interested in learning the percentage of UK Business email
accounts that do not accept HTML emails from other companies compared
to Text. We regularly carry out B2B email campaigns within the full
guidelines of the UK DMA - The DMA is the UK body that sets the rules
on B2B email. We only ever market to UK companies our services.

I don't have an answer to your question, but will offer you a thought.

No matter what the percentage, do you want to risk your HTML formatted
email to look broken, unprofessional and possibly incomprehendable for
those receipients who have their client to accept only text based
email?

You could come to the conclusion that 1% (of whatever the percent
turns out to be) is neglible.  The part I'll point out is what if that
1% that got this email was destined to be your most important
customer.

I would not take the chance, if it were me.  If you design for the
lowest commont denominator, text in this case, nobody will be
disappointed.

Food for thought.

It is a thought I have considered, however before I can make that
decision I want to know the percentage of email clients that will or
will not accept HTML