I was in the Harlow Sainburys Earlier this morning and I noticed they had these smoke alerms reduced from £4.99 down to £1.25. They are basic alems with a button to test the battery. They were with all the light bulbs in the store, so may be in the same sort of place in other stores.
The picture I have used is the closest one that i could find on the net to the one that is in the store, but it may be slightly different. I did not buy one as I have enough, but wanted to post as I though it was a good deal.