Apparently there's a rumour, that the older Exec drivers (who joined probably 2013-2014), who currently accept Exec work only, will have to accept X work sometime in the New Year. I can imagine a lot of those drivers (myself included) leaving.
To do Exec work in a Lux car, at £1.80 a mile and 22.5p/minute is a joke though.
Probably why they're inviting a lot of X drivers to upgrade to Exec/Lux - those with 1,000 trips under their belt, with a 4.85 rating or above. They know that forcing the existing Lux drivers doing Exec, and Exec doing X, are going to leave, and need to make up the shortfall.