Benefits continue even when on maternity
"Under the Maternity and Parental Leave Regulations 1999 employees are entitled to all their terms and conditions of employment, apart from remuneration, during ordinary and additional maternity leave. This means employees are entitled to continue to accrue benefits such as paid annual leave during 52 weeks maternity leave."
Having said that, it doesn't seem to be 100% clear whether something which is effectively a cash benefit should be counted as "remuneration" or a benefit. It's possible that BT were being more generous than they needed to be prior to this change, but with the car benefit being rolled into salary, it's pretty clear that it counts as remuneration & as such would not be paid during maternity leave.