Pure speculation on my part, but suppose the 251 number story was the other way around or purely coincidental. Suppose the company didn't want to start numbering chassis at 1 (After all, who would want to buy car number 1 of a new model?) So perhaps they arbitrarily decided to start numbering after say 250. After the move to Abingdon, I find it unlikely that anyone would thing of asking BT to give them a number that matched the first chassis number of the cars.
The Abingdon 251 'phone number would have been allocated by the GPO (General Post Office) . BT was created about 1980. The telephone number looks like coincidence but a 1929/1930 'phone book for Abingdon might provide a clue.