Correct. If you have an ICAO commercial licence and current ICAO class 1 medical, i believe you will still have to go to Gatwick for your first UK (JAR) class 1 medical but it will be what they call an "extended renewal". This does cost considerably less than their initial class one (£411) because there is no EEG, and possibly a few other tests. However, the cost is still high by FAA standards. I had my initial class 1 at Gatwick in September (£411) and a chap with an ICAO class 1 medical certificate was having his "extended renewal" at the same time. I think he paid between £200 and £300, but I can't remember the exact figure.
Thereafter, in the UK the JAR class one renewals can be done by any Aviation Medical Examiner and they are dotted around the country. Typical class 1 renewal costs are £100 to £150 I believe.