Royal Mail wouldn't take it built and I would NOT recommend sending it built. RM would also only take it as one parcel if it was under 20kg total and met their measurement criteria. See:
http://www.royalmail.com/portal/rm/content1?catId=400031&mediaId=51000711
..for info. You could always send two parcels, of course, but I'd suggest you need to allow £25 for p&p which includes your time and effort to get it to the PO.
Parcelforce can be arranged through eBay:
http://pages.ebay.co.uk/postagecentre/index.html
and you should be able to package it as one item.
For, say, 22kg they quote around £25 for a two day service which offers tracking and a bit of insurance. You still need to cost in the packaging part though.
Packaging tip: If you live near an industrial estate search the bins for decent (industrial strength) cardboard/boxes/foam/bubble wrap that has been thrown out. Buying packaging new costs a fortune and if you don't package it properly it isn't likely to make the journey as it won't be handled any more carefully than any other parcel, just because it's falling to bits.
Don't just put the parts in a box; pack them in the box so the don't rattle/move. Moving items within a box (like an axle) are more likely to punch their way out of a side/end and get lost in transit. Pack the item with the expectation that it will be thrown at some point, as it probably will. It is likely to be handled by several people, none of which will be interested in the contents and will handle it accordingly. (That applies to ALL carriers!!)
Make sure small components are either bolted into the frame or are in a sturdy bag that is taped securely to the frame. Protect corners not only from damage but from injuring the handler.
Don't be shy when using parcel tape. More is good. Expect at least 5kg+ added to the net weight when packaging something of this size.
Of course a 'high' postage cost (albeit unavoidable) may affect the sale price as buyers would take it into account when bidding. However, if you're prepared to pack it properly (and state that in the listing) and the buyer is prepared to pay for it then you can pretty much guarantee it getting there complete.
Hope that helps.