Read the lease before you book anyone
The lease decides almost everything: whether you have to reinstate at all, to what standard, by when, and whether the landlord can do the works and charge you instead. Find the reinstatement clause, the handover provisions and anything on the security deposit, and ask the landlord or building management for its reinstatement specification or fit-out guide, which usually sets out what “bare shell” means in that building.
Clear in the right order
Deal with anything that holds data first: filing cabinets, archive boxes, laptops, phones, servers and the hard drives inside printers and copiers. Then decide what moves to the new premises, what can be sold or donated, and what has to be disposed of. Only then should the strip-out start. Getting the order wrong is how a box of personnel files ends up in a skip.