Cruises - Rental car in Harwich, UK

I will be arriving in Harwich, UK, at the end of a transatlantic cruise. I will be spending a week in the UK before returning to the states. Does anyone have any advice about renting a car there? It looks like I will have to get to Colchester to rent a car. What do taxis cost there?

Europcar and National are local and offer free delivery. Visit the website for more details.