We have a two tier level of local government in Surrey. Surrey County Council looks after some stuff such as highways, hospitals and schools, whilst the 11 Boroughs looks after planning, housing, community services and such like.
Implementing a unitary authority combines these two levels of local government. It isn't unusual, 25% of the population of England already live in areas managed by unitary authorities. There are around 50 of them across England and vary in size from around 50,000 to about 500,000.
Surrey County Council is supporting the idea of a single Unitary Authority across the whole of Surrey - that's 1.2m residents so more than double the size of the largest existing UA. Upside from SCC's point of view is the maximisation of economies of scale and savings. Downside is the democratic deficit (i.e. the distancing of democracy from residents).
The 11 Surrey Boroughs don't support a single UA but some sort of hybrid model - maybe two or three UA's in order to minimise the democratic deficit but also achieve cost efficiencies.
Here's a link to a press release by the Boroughs https://www.runnymede.gov.uk/article/19564/Campaign-launched-to-transform-local-government-across-Surrey
Over the coming months expect this to be a lively debate!!