It is extremely rare to virtually non-existent for a person to be considering these two speakers. They are very different speakers both in very different classes.
The B&W are expensive and refined, and the Xarus 5000 are low cost, loud, and heavy.
So, you need to ask yourself, exactly what it is you are looking for? If you really want to rock the house and raise the roof, then the Wharfedales are probably right for you. But, if you want a more sophisticated, refined, and expensive speaker that will still have fair bass, but is not likely to demolish the house, then the B&W 600 series are for you.
The smaller speaker will do a better job in a smaller room for more casual listening, and still accomplish a fair party. The larger speaker will make some serious ass kicking parties, but will likely be somewhat overbearing in a small room for casual listening. Though still functional. But rarely does one speaker do both equally well.
In theory, you might be able to find some USED B&W 685, but I wouldn't hold much hope for it. A quick search of Ebay-UK only turns up one pair, and they are £319/
pr. So, unless you specifically know of someplace that has a good used pair of B&W 685 for sale for an extremely modest £150/
pr, you really don't have the choices you thought you did.
Now, if you can raise your budget to £400 more choices open up.
Amp wise, 56watts is enough to get the job done with either speaker. We all want 100's of watts, but for practical and financial reasons, we all settle for 10's of watts.
Do you specifically have a a pair of B&W 685 lined up for roughly £150/
pr?
Steve/bluewizard