Shell UK said yesterday it planned to invest £10 million sterling to offer an alternative fuel for cars at some 200 petrol stations throughout Britain. The Anglo-Dutch Shell Group unit said the sites will offer motorists liquefied petroleum gas (LPG), which is cheaper and produces less pollution than standard petrol. The first of the sites opened yesterday at Dover, with a further 25 expected to open over the next five months, followed by 100 more by end-1999.