It depends really what you mean by TOCs owning buses.
Taking it literally, then I am fairly sure that no TOC (e.g. South West Trains / First Capital Connect) either owns buses, or holds a bus / coach operators' licence.
Similarly, the big groups, e.g. First plc / Stagecoach Holdings also do not directly own and operate buses or coaches - these are run by subsidiary companies e.g. Stagecoach South etc - the name on the operator disc and the name / address on the side of the bus / coach is the actual owner of the bus.
There are a host of reasons why the big groups have operations under a number of subsidiary 'bus companies' rather than have one huge, national legal entity.
It is not uncommon for some bus operators to have vehicles in other liveries, e.g. National Express (most but not all 'National Express' coaches are run under franchise by other bus / coach operators, the contract between Nat Ex and those operators generally requires branded vehicles), some coaches that run in the livery of holiday companies are owned and legally operated by a PSV operator not the holiday company. I think they are getting rarer, but I've seen buses running in Asda livery - again the vehicle is owned and operated by a particular bus company.
Within Stagecoach, there are vehicles which are legally operated by (for example) Stagecoach South, but are in SWT livery.
The TOCs have some sort of department / office / people who plan and organise rail replacement buses. The larger groups' TOCs tend to put this work through their own subsidiary, e.g. First Rail Support -
http://www.firstgroup.com/firstrailsupport/index.php, Arriva UK Trains road transport section -
http://www.railbyroad.com/
Those rail replacement companies tend to employ bus co-ordinators (who may or may not have other jobs in the transport industry and / or within that group) and they plan services and procure buses and coaches to run rail replacement services. They may do this via other subsidiaries of the same group, they may go elsewhere - I have seen First London vehicles (when such things existed) on FCC work, I have also seen a variety of coach operators and Abelio London, but I can't recall seeing First Berkshire running around Reading, though.
Go-Ahead do it slightly differently in that the planning for Southern / South Eastern is done by Go-Ahead London (bus company) and they tend to use their own buses (and Metrobus / Brighton & Hove) where possible, sub-contracting whatever work they can't cover.
