From Memory, (I don't have a sectional appendix in front of me) 165/166's can work to Bristol from Swindon (not sure if that is via both routes).
But are not permitted Bristol-WSM or Bristol-Westbury.
But they are permitted Westbury-Taunton-Exeter.
If true, this would be very interesting, as Jacobs Consultancy proposed running Turbos down to Weston as an extension of their Bristol-Oxford service option, which was rejected by the
SRA» (link below.)
http://www.savethetrain.org.uk/lf/CoIUTS.xlsHere is how Jacobs saw it (bearing in mind that they wrote their franchise reports in 2004) :
This option tests extension of some Oxford to Bristol services to Weston-Super-Mare, replacing the through train from Avonmouth in that hour.
This option enables the busy 0735 Weston-Super-Mare to Bristol to be operated by a 2-car class 165 unit and the 1727 return services to be operated by a 3-car class 165 formation. This offers both an increase in capacity over the 2 and 4-car class 143 respective formations employed on these trains and the opportunity to withdraw completely from one class 143 strengthening diagram. Overall the option saves one class 143 at the modest additional cost of one class 165 2-car diagram. The additional class 165 diagram is however able to be additionally utilised to operate the new morning train from Bristol to Oxford.