Papergirl,
Ask them what colour/type of wood they want done. If they can show you look at the palest colour in the timber then match that as a basecoat. Paint the beams in this colour in eggshell. Go to the suppliers and buy Ratcliffes oil scumble, less than £5 for a 250ml tin which will go miles. It comes in a miriad of wood colours. Mix it up in the tin because it settles badly, thin it. You can just thin it with white spirit or you can do as I do, make a mix of raw linseed oil 1 pt: white spirit 3pts + 10% driers and add that about 1:3 scumble to mix...my method just gives you more time to play. Lay it on really thinly, you need to keep mixing it up as it settles in the pot. Then take a dry brush and draw it back through the scumble, play with it till you get a nicely woody look. This is basic brush graining, not difficult. If you need further info feel free to ask, it really is very foolproof. On beams I think that if you can get the right colour and texture that you need do little more than a brush grain as very few folk would look up and say "gosh, oak and no heart grain". There are lots of other graining techniques I could tell you about but this is a good start.
If they can't show you a wood they want or just say something like "medium oak" come back to me and I'll give you a good base coat colour.
I'd do them a wee sample before you start just to let them see the colour and to let you play with the medium.
It needs varnishing to even out the sheen and protect, but on beams if they are happy with the look without varnish then you needn't bother as it will get no wear.
Cait