Has anybody told the owners of the Woodworm Express about this 'ban'?
http://paulbigland.pikfu.net/set130252/media30763935.html
As Snapper has rightly said, the "Queen of Scots" set (although I like the "Woodworm Express" title) mainly consists of some pretty ancient pre-grouping rolling stock, including a couple of Victorian era vehicles one of which is a "cut and shut" job containing the cab of a class 101 DMU acting as an observation saloon (It saw use in the original "Royal Scotsman" train IIRC).
I have no idea what sort of restrictions are placed on the operation of the Queen of Scots carriage rake, but it shows that it is possible to operate older vehicles on the national network. But as far as seeing a full rake of Gresley teak or LMS Stanier stock on the main line is concerned, unfortunately I think it is unlikely, especially in view of the much increased standard of maintenance that would have to be lavished upon the vehicles to make them fit for 75mph running on railtours.
Although if it was to happen, I'd rather see a rake of Stanier LMS stock on the main line (A truly blasphemous proposition from such a staunch LNER man as myself), as from my experiences travelling in both types of carriage the Gresley stock seems to be rougher riding even at 25mph, as much as it pains me to say it.
I may be wrong, but didn't EWS operate a Stanier design (although built in BR days) inspection saloon into the early years of this millenium?