N.B. you have the right to make more than one phone call
if you need to, though when you get to do this depends on the vagaries of the desk sargeant
If you are arrested and you've never been arrested before/don't feel confident about dealing with the cops, make sure you ask for a solicitor as the cops are not legally allowed to question you until they arrive. Don't accept the duty solicitor if you can help it, phone Release, or alternatively there should be a list at the front desk. Above all Don't Panic, and don't say anything until you've seen the solicitor. Being arrested isn't so bad, mostly its just very boring.
A common tactic for the police if they turn up is to try and arrest you for Abstracting Electricity - it is a defence to prove intention to pay - you do this by taking a meter reading and then signing on for an account at the earliest opportunity. If they try and nick you for this (most often its a way of bluffing their way inside) tell them you have the reading and plan to go to the Electric Board first thing in the morning.
The attitude of the police to squatters varies widely from area to area but often once you've convinced them you are a squatter they will go away as long as there is no obvious sign of damage.