UK Question. Can anyone recommend a good security company in the Sheffield area?

ME

Define good (or maybe what you call bad)
and what is that you expect them to do...

Just walk the site out of hours
Be on the front desk / Validate and Admit people into building
more than just that ?
Walk the inside of the building ....

Security companies should already be part of an industry acreditation,
whilst a standard, it may not actually be more than self-serving/self-supporting industry body.

There's always G4S !

It seems from all that you are asking, that you should really have joined and met the local FSB. Lots of local traders (and discounts / introductions to trades and services - including business insurances)

THEY should also have the local knowledge that you are asking for
http://www.fsb.org.uk/

or the (Sheffield) chamber of commerce
https://www.scci.org.uk/

FSB: South and East Yorkshire Region
http://www.fsb.org.uk/regions/south-east-yorkshire
http://www.fsb.org.uk/regions/south-east-yorkshire/branches