can't comment on the
XC▸ Voyager loadings from Bristol. As I avoid them where at all possible I have no idea what the 1844 from Bristol will be like. However if you do go for this option on the (current) ^22.00 ADV you will get a seat reservation. Book your ticket via
crosscountry.co.uk and you can also choose your seat on the Bristol to Liskeard leg.
As regard the Pullman Restaurant on the 1803 ex Paddington, this service is only officially available from Paddington and Reading according to the timetable. It is usually well used from those points, even on a Weds, so they are unlikely to offer you a seat in the restaurant from Taunton. However, that being said, if it has been quiet from London you may get the opportunity to have a meal; I've seen people board at Taunton in the past and been served. No guarantees though - depends how desperate they are for the custom and what stock is available.
On the Pullman services there is just a standard buffet for the proles!
Another option if you can leave work early enough: Take the 1536 from Bath and change onto the 1623 at Westbury arriving Liskeard 1906. Current available ADV fares are ^23.00 Standard or a bargain ^29.50 1st Class. Plus the 1623 from Westbury has a Travelling Chef with at seat service if you choose the 1st Class option. I fear this train maybe to early for you though!