Please keep all ativities legal. It is best to contact local police first. Be prepared for delaying tactics, however.
In England Islamists seem able to demonstrate at a moment's notice. SIOE doesn't know whether this is because Islamists receive more favourable treatment from the authorities than other people, or whether Islamists just show up at a venue and get away with it.
Just such a demo happened outside Westminster Roman Catholic Cathedral the day after Pope Benedict's observations on a Medievil text
SIOE England considers that counter demonstrations outside Islamic centres in just as short a space of time should happen with as much ease as the Islamist demonstration referred to above.
Therefore, here is a list of Islamic centres in the British Isles
Regents Park mosque
The Islamic Cultural Centre & The London Central Mosque
146 Park Road. London. NW8 7RG
http://www.iccuk.org/icc/location.htm - for a map
Muslim Council of Britain
http://www.mcb.org.uk/ - location and map required so please post them
London Metropolitan police contacts
UK police stations