Schimmel,
If you renew Class 1, you automatically have Class 2 privileges, so no need to do a separate class 2 medical.
All costs for medicals in the UK are set by the AME individually so it is worth shopping around - I paid £60 last year for a class 1 and £110 the year before!
You can go to any AME that does Class 1 medicals to renew your class 1. There is no difference if you last held a class 1 some time ago or are just renewing normally.
If you need to know more, this is the medical section at the CAA website:
http://www.caa.co.uk/default.aspx?categoryid=49
See especially the section 'find an AME' to locate a doctor for you.