The UK government provided a lot of the money to AZ to build out the local vaccine supply chain and factories. Those factories most likely wouldn't exist unless the UK government had paid that money. So, as part of paying for all that, they asked for timed exclusively.

Also remember the vaccine was developed by Oxford using a significant amount of UK government funding, and was licensed to other countries, such as India, for low cost manufacturing.

We did also buy a lot of vaccine from other places (e.g. Pfizer which is also being widely deployed). But with AZ in particular (and to some extent Novavax) there was a focus on building a local supply chain so that countries can't play games with our vaccine supply. Supposedly Pfizer turned down the offer to build UK based factories.