I don't know how it works in the UK so couldn't come up with answers that best suits you. In the US, college affiliated with Council of Graduate Schools (CGS) let their accepted students make decisions by April 15. Students are under no obligation to offers of financial support prior to April 15; and, earlier deadlines for acceptance of such offer violate intent of the resolution.
You might want to see if there are any such resolution enacted to protect students interest by UK law.
This will let you hold the best offer time until certain time. If you get a new offer from somewhere else you can compare the latest offer with the one in your hand and make decision accordingly.
