I saw that the same program, Rob. I think htey only used reed matting because the couple wanted "original techniques"; they also used lime render. I doubt it's needed - or wanted - here as these aren't oak beams being reclaimed from beneath boarding. Also the cost would be a factor; reed matting ain't cheap! PB is a lot cheaper and will do the job.
As for the beams...the best option, though messy and NOT cheap compared to the alternatives, would be to sand blast them. But not really a "goer" for a quick DIY fix. So I'd sand them back.
If you haven't already placed the plaster board in situ you might sand them first so that you can get a good sand on the inner sides. If the 6 by 4's are pretty even, which they ought to be, you might find a belt sander an advantage...just don't let it sit in the same place...keep it moving or it'll eat a trench into the timber quicker than a family of woodworm.
Your eventual finish is up to you...options:
1) Varnish...not my choice, but if you do then I'd go for a matt or semi-matt water based laquer.
2) Wax...a good finish but if you're doing a lot of timber could be expensive at about £6 a tin for decent wax.
3) Oils...tung oil or danish oil would be my choice. They protect, nourish, and enhance the grain and these two wouldn't need the timber sealing all over first - although still seal the edge faces near the plaster.
Rustins do a range of wood finishes that I've always found to be very good.
http://www.rustins.co.uk/
