Re: UK microlight on FAA PPC?
You may use the privileges of an ICAO licence (such as a FAA PPL) to fly SEP Class aircraft in the UK under day VFR conditions. All UK night flying is IFR except for SVFR in Class 'D' airspace; your night flying privileges elsewhere are unclear and open to interpretation due to the FAA requirement that to fly under IFR you need an IR.... The UK does not have such a rule, provided that you do not fly in IMC.
Once you have converetd your licence to a JAR-FCL PPL(A), you will indeed need to complete differences training to add microlight privileges to your SEP Class Rating privileges. As for cost, excluding hire costs a PPL Skill Test will cost you about £160 for the examiner's fee, the licence issue fee is £159 and the licence lasts for 5 years, after which a £63 re-issue fee is required. To maintain validity, you must either achieve the required experience in the second of the 12 month periods for which your Rating is valid, or pass a Licence Proficiency Check in the final 3 months of the validity period. Microlight hours do not count towards revalidation.
'Group A' is an obsolete term and referred to single engine aeroplanes of less than 5700kg MTOW which were not microlights. The correct term is now 'Single Engine Piston' Class - SEP Class for short.
To learn about UK differences, download CAP413 and LASORS2006 from the CAA website. Then you will no doubt be required to fly with an instructor before people will be prepared to hire you an aeroplane.