Given that the combined postage option hasn't been working for months it wouldn't surprise me if eBay has done away with it for good yet haven't had the *bleep* to communicate this to their users, as they know that it would be a vastly unpopular move and would more than likely spark a furious backlash from buyers and sellers alike.
With regards to the Best Offer option not working, you're not the first person to report that there's been a problem with this function, as I recently answered a post on a separate thread about the matter. What I suggested on that thread was to log off of eBay UK and log into one of their overseas websites in order to see if it was an eBay glitch specific to the UK site. Whenever I've had to log into eBay via an overseas site in order to bypass glitches that only seem to be affecting the UK site I've logged off and logged onto the USA version of the site. Therefore, if you haven't already tried it by the time you read this response, save the item in question to your Watch List if you have not already done so, then log off of the UK site and log in again via www.ebay.com, which is the American version of eBay. Once you've done that locate the item in question on your Watch List and try submitting a Best Offer for the item that way.
If after doing that the offer goes through successfully then it may well be a glitch that is only affecting the UK site. However, if you experience the same problem when attempting to submit a Best Offer via one or more of eBay's overseas websites then perhaps eBay are in the process of trying to implement some more of their badly thought out "improvements" to the site, the end result being that they have screwed up yet another aspect of the site that used to work perfectly!
