All government procurement over a certain value in the UK must follow EU regulations around transparency and fairness.
The NHS is no different, and so healthcare contracts are subject to EU procurement law. The threshold values for procurements are updated regularly to keep them in line with inflation or any other legislative change.
The 2018 values have been confirmed as follows:-
Supply, Services and Design Contracts - £118,133
Works Contracts - £4,551,413
Social and other specific services - £615,278
These values are for the whole contract lifetime, and exclude VAT.
The deifinition of a social service is complex, and is based around CPV codes. A list of these can be found here.