bathroom windowsills are prone to damp and shampoo bottles which will mark them. An oil based gloss system will protect them better.
Otherwise, I'd go for a satin varnish such as Ronseal Diamond. The one sold for floors is extremely hard wearing. If you want to colour the wood first, tidy it with extra-fine sandpaper on a cork block, clean with white spirit, and dye it with spirit-based Colron Wood Dye. This will give a better colour and texture than using a tinted varnish or stain. It is available in a vast array of colours from large DIY or paint merchants. Cheaper substitutes may be almost as good.
You must protect the wooodwork while painting the walls and ceiling. It is easier to clean fresh emulsion off varnish, than to try to get varnish off walls, so do the woodwork first.
Had new ensuite now for 3years and still not decorated it!! Do I need to prime the plastered walls first with a thin emulsion or will it be ok to paint straight on to them. Is there a special roller brush which will go behind towel radiator? Want to leave wooden skirting and window sill so what stain should I get? Thanks!