Both Vodaphone & Orange can do this.

Orange = "Single Number"

Vodaphone  = "Dual Sim"

http://www.orange.co.uk/business/corporate/using/callmanagement/singlenumber.html

You still pay for 2 sims on both. The Voda has limitations in that no fancy
tariff sharing is available, nor MMS etc.

With the Orange option, you choose which phone is the main handset at the
outset - this phone is the one that all texts are delivered to.

Why not just activate call diversion on the handset you don't use in the
car - choose divert when not available & divert it to the car phone number.
then, whenever you're in the car, simply turn off the other phone. Also,
withhold your number on the car phone.