The law is fairly clear. You have to wear a mask in the same places you aren't allowed to smoke.
For the purposes of these Regulations premises, or any part of premises, are “indoors” if they, or it, would be considered enclosed or substantially enclosed for the purposes of section 2 of the Health Act 2006 under the Smoke-free (Premises and Enforcement) Regulations 2006
The law is fairly clear. You have to wear a mask in the same places you aren't allowed to smoke.