Traveling from Heathrow to Brittany Ferry at Portsmouth in June. Reliable transport suggestions?
Coach would probably be the best option:
https://www.londontoolkit.com/travel/heathrow_southernengland_transfer.htm
I've used these before http://www.southcoastexecutivetravel.co.uk, good prices and service.
http://www.aquacars.co.uk are Portsmouth's largest taxi company and offer good rates for airport runs.
A saloon car should cost around £60 - £80 so depending on how many there are it could work out more cost effective than the coach.