I have five 100 US$ notes, 2008 series. Can I change them to UK currency in the UK?
They are in perfect condition.
Yes you can.
Unlike other countries, including the UK, US banknotes are never "withdrawn" and remain legal currency across the world however old they are (but you wouldn't want to spend the older ones as they are worth more than their face value as collectors items).
In the UK you can exchange them at most high street banks (NatWest, HSBC, Lloyds etc), any Post Office with a Foreign Currency desk, many travel agents (Thomas Cook etc), Marks and Spencer stores and high street bureau de change (currency exchange).
You may be charged an exchange fee, and you should shop around for the best rates.