I think they should be entitled to ask politely but should graciously accept a refusal. Much of it is now 'door2door' as it doesn't need the same licensing as High Street.
Now that Direct Mail and TeleMarketing have run out of steam it is charities main source of new donors though attrition is very high including a large proportion of 'Non Starters'.
Most use specialist Agencies to do this for them except for Greenpeace who employ their own people who are committed to the cause.
A shake up in the law overall could be imminent, largely prompted by concerns over doorstep collections of clothes etc that are 'going astray'.