First welcome to the UK, sorry about the weather. If you are arriving from the US then you’ll probably be landing at Heathrow or Gatwick airport near London. Firstly, don’t think about renting a car if you are staying in the centre of London, traffic is a nightmare, there is congestion charge of £11.50 a day, and parking is very expensive. The standard of driving is also not the best (just think of Ben Hur). Public transport is your best option for this part of your trip.
If you intend to explore the rest of the UK, especially the more out of the way places then a car would be your best option so here are some completely unauthorised but practical tips and quirks about driving in the UK for the first time.