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We've had tags (even quite far? one or some of rhowe's IIRC) outside the A406 before. As long as it's not taking the piss, you'll probably find someone willing to ride there. In South London, the South Circular is obviously less far south than the North Circular is north, and I'm sure we've had a lot of tags outside it.
Perhaps it's time to change the old rule. It seemed sensible at the time but perhaps no longer?
I suspect your tag would be fine even while violating the rule.
A query:
What are the current accepted limits of BTOB? I have found something that I dearly want to tag, but am not sure how far from central it is OK to venture?
For reference, the location I have in mind is outwith the N/S circulars, within the M25 and circa 12 miles from Charing Cross, as the crow flies.