Man, don't do anything to hurt anyone, you will be liable, even in the UK.
Famous case here called "Katko v. Briney," homeowner rigged up spring-gun, blew leg off intruder.
http://www.law.mq.edu.au/Units/law405/Study_Guide/Katko_v_Briney/katko_v_briney.htm
Anyway, add up the amperage draw of all of your stuff. If you don't have a clamp-on ammeter, look at the amperage draw on the power supply for each thing. I would think the IR illuminator would be the biggest draw, unless it's LED. That should help you size the battery capacity in amp-hours. Remember that you don't want to fully discharge a regular (non deep-cycle) battery, that will kill it. Check the hawker battery site for some good information about how far down you can pull the batteries.