My parents were in a similar position last year after over 3 years of no activity

They emailed BA via the website and explained the position and requested the miles be re-instated

BA were very helpful and arranged this quite quickly

They had a new trip booked on BA at around the same time (this was after the 3 year expiry date and after removal of miles) - so it may be that the pending activity may have helped their cause).

Best advice - request the miles be reinstated from BA and explain that they didnt know the rule