It's not simply 5x HST sets that Great Western Trains Company Ltd (trading as First Great Western) own outright.
There's 10x Class 43 power cars and 36 Mk3's of varying types:
5x TRFB
3x TF
3x TFD
18x TS
2x TSD
5x TGS
With only two Standard Class disabled carriages owned outright, it would be difficult to put 5 full 7/8 carriage HST sets into service elsewhere in First Group's rail division. A few shorter sets is possible, but I suspect the return on leasing the entire owned outright fleet to a new operator would yield a better return than transferring the rolling stock to another First Group TOC. As others have mentioned, there would be track access charges, route clearance, staff training, maintenance facilities, etc, to be provided for.