dose your dog need to be reg with The companion dog club to enter a companion show?

No. The only dogs that can't enter companion shows are Champions. Any other dog, whether pedigree, crossbreed or mongrel, registered or unregistered, can enter. (Obviously only pedigrees - not necessarily registered - can enter the pedigree classes, but any dog can enter the 'fun' classes.)

Only to enter the Companion Dog Club classes, i assume. Only certain Companion shows have these classes anyway. Regular Companion shows are usually split between Pedigree and Novelty Classes. For the Novelty classes, the dogs do not need to be registered.

JW holders also can't compete.

The only dogs that can't enter companion shows are Champions.

Plus JW, CC and also RCC winners -BUT they can still go in the novelty classes, it's just the pedigree classes they can't enter. http://www.thekennelclub.org.uk/item/1824