1) Not really, it wont stop you getting employed by a UK operator, although they may ask you to convert to JAR CPL. If so, it just a paperwork exercise, and a fee...of course. Having JAR CPL may make you more employable in other JAA states though.
2) Of course. You have to, as there is no Perf A exams anymore.
3) UK CPL (A)/IR holders who have passed UK ATPL (A) theory examinations, and have 500 hours flying experience in multi-pilot operations will be credited the JAR-FCL ATPL(A) theoretical knowledge examinations for the purpose of JAR-FCL ATPL(A) licence issue. This credit will remain valid for a period of 7 years from the most recent validity date of the IR (A).