There is enough info on Challen in the Piano Atlas that I suspect there is someone on this board who knows about them first hand. For what it is worth, here is what the Atlas has to say:
"Challen Pianos, Chas H. Est 1804 by Thomas Butcher. William Challen entered the firm in 1816 and in 1830 he took over the company. The name was changed to Challen Pianos, Omega Works, Hermitage Road, Finsbury Park, London N4, England. Taken over by Barrett and Robinson Ltd. in 1971. In 1984 John broadwood and son took over the firm. Musical Products Sdn. Bbd. of Malaysia started building pianos with the Challen name in 1996.
There is even a picture of a Challen that is is billed as the "Largest Grand Piano in the World" . It's 11'-8" long, 2000 lbs. Looks more like a freeway bridge than a piano.
There are serial numbers listed from 1850 through 1983, so if you can supply a serial number, I can tell you the year in which it was made. If it is indeed a 20's era piano, it probably was made by Challen. Later pianos (according to the history, above) may say Challen on them but be made by someone else. This might explain why you see them for sale ont he net - the name has been around for a long time.
As for buying one as a gift, you would be wise to engage the services of a piano technician to give it a going-over before you buy it. I think old pianos are wonderful things, but (assuming you intend to play it as opposed to admire it), there are a lot of them out there that look good but are beyond economical repair.
Hope this helps.
Defender of the Landfill Piano