while i admire the decency of many folks on this thread, you are perfectly entitled to make an offer on any house at any time, including when it's not even on the market.
an offer amounts to little more than asking the question "would you be interested in selling your house to me for X amount?"
yes, if the property is already under offer, then someone may have already forked out for surveys & searches, but in england, thats just the way house buying/selling works - like it or loathe it.
do bear in mind, though, that if your seller jumps from an existing potential purchaser to you, because you make a more attractive offer, then they could also do the same to you, if someone even more appealing comes along...and that would be rather karmic.
you could also find yourself in a bidding war.
don't get me wrong - i'm not a fan of the system, and i have also never offered on an "under offer" property - we have just, however, gone through the agonizing process of buying a repossession, and not only did the property remain on the market after our offer was accepted, but they ramped up the marketing significantly, including the publication of the exact amount of our offer in all the adverts, to try and solicit additional offers. during this period we spent several thousand on surveys, searches, inspections etc, all of which was done "at risk".
we spent a lot of time praying that noone came along and snatched the property from under our noses.
they didn't, but it was a horrible period. i've no idea whether the ethics of offering on an "under offer" property deterred anyone from offering, but if it did, i'm delighted. in the highly pressured housing market of the south east, i suspect those people would be in a minority, though.