I haven't got the SA to hand but last time I checked that line was indeed 100MPH (it stuck in my memory because as far as I'm aware it's also the highest line speed on any single track section in the country) - however, AFAIK it's Oxford and Bristol who sign that route, and they both go to London and therefore can do 125 regularly enough. It was mentioned earlier that Gloucester sign Didcot-Swindon as a diversion, which gives them a very occasional run at 125MPH, is that because they sign the Cotswolds line as well? And I wonder do any of the Paddington HSS drivers go that way as well - albeit again they'll see 125MPH regularly.