You should not plead guilty simply to get the matter out of the way. If you genuinely believe (after seeking legal advice) that you are not guilty, then that is how you should plead. Bear in mind that the �Highway Code� stopping distance from 40mph is 120 feet (less than 37 metres). From your description of events the police car was well beyond that distance when you pulled out and should not have been unduly jeopardised or even inconvenienced by your manoeuvre. However, that is a matter for you to decide.
As you are being asked to appear on summons (as opposed to on bail) you may choose not to appear but to plead guilty by letter. Your penalty will not be affected by doing this.
Make sure that you clearly indicate your guilty plea and mention anything else that you want the magistrates to hear either about the offence, or about you personally (in particular your financial means if they are limited). If you wish to pay any fine by instalments make a (sensible) offer, otherwise you may find that you are simply given as little as 14 days to pay in full.
Your letter will be read to the Bench by their legal advisor (the �clerk� as most people know them). Bear in mind that as a result of your guilty plea the magistrates will accept the prosecution version of events as fact.
Finally, the Bench may adjourn the case and ask you to appear in court before they pass their sentence. They will do this if they are thinking of imposing a disqualification for the offence. Again, you do not have to appear, but it will be your only opportunity to explain to them the effect that a disqualification may have upon you and to persuade them to allow you to keep your licence.