From what I see in rural Cambridgeshire the unofficial installed flags are used more for provocation than patriotism. If used on a road the Highways Act 1980 s132 considers it an offence with a fine of up to £100 (first conviction) or £200 (second or subsequent conviction).
If you had a whole group of people putting out flags and they coordinated their actions, the sentencing guidelines (for “conspiracy to…”) are harsher. Especially relevant if the judge finds that encouraging hate crime was part of their motive.
The hate crime bit is interesting. The flags near me have been placed on a roundabout with two adjacent business, a petrol station and a hand car wash. The staff at these business are not "White British" [ONS Definition] so it seems more than a coincidence they've been put only on this roundabout.
It'd be a fascinating social experiment to go and install a Council of Europe flag, or even a rainbow flag on the same lampposts. I wonder how our elected representatives would deal with this?
Good to know. Thanks.