Currently, 'long final' means >4 miles in the UK. 'Short final' may also be heard and though it has no official definition, use is tolerated as it is generally understood (i.e. between 'final' and 'over the hedge') and may help a pilot understand more about traffic position.
In practice, small aircraft may wish to join final that far out for all kinds of reasons and I don't see why they shouldn't if it fits in with the pattern of traffic. The service I operate asks any traffic (overflying or joining) to call if within 10 miles of the field, as this strikes a balance between providing an efficient service and not stepping on other airfields with the same freq.