Uber are offering a free rides promotion. This is an example of the Nottingham deal but other cities available and valid on different days and date ranges throughout August.

"Free rides in Nottingham every Monday in August

Since launching in Nottingham in November, Uber riders have traveled more than 500,000 miles together. To celebrate this milestone, for the first time ever, Uber rides on Mondays in August are now free up £10.

That’s right—starting on Monday August 8, anyone who requests a ride in Nottingham on a Monday will get up to £10 off up to 2 trips that day—whether you're a new or existing Uber user!

Whether you're catching a film in Wollaton park, commuting across town, or planning your next bank holiday, your Mondays just got that much better.

How to get your free rides*:

1. Open the Uber app anytime on Monday, August 8, Monday August 15, Monday August 22, and Monday, August 29

2. Request an uberX or uberXL

3. That’s it!"

Known available cities:
Nottingham - Every Monday in August
Bolton - 8th-12th August.
Bristol
Sheffield
Leeds

Note: The city info has been crowd sourced on this forum, post on this thread if you spot any more or if you spot any errors and I'll add/amend them here.