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Yeah, i think chickens are definitely out,
besides anything else, their housing ends up infested with red mites (no matter how hard you try, as at some point they will just appear) and their crap smells appalling.

Beardies are wicked, the initial cost of an enclosure & equipment for them is a bit steep though.
a 3ft x 1.5ft vivarium is enough for one adult dragon, but bigger is always better.
then you'd need to set up a basking spot, so that would need a heat bulb and a thermostat to regulate the temperature as well as a guard to stop your dragon from burning itself
you need to have a tube in there that supplies UVA and UVB lighting, and those tubes need to be replaced on a yearly basis as they tend to become less effective as time goes on.
feeding wise, they're quite cheap, you'll maybe buy a box of crickets or locusts every week or two and then give them a bit of salad along with it.
it's usually a good idea to dust their food with a calcium supplements as well.

i don't know a whole host about them, but as a rough guess for the enclosure and all, you're looking at around the £150 mark to get started at the very least. maybe £200 after decorations as well.
food wise around £10-20 a month.

if you still want a lizard, a leopard gecko could be a good alternative, as they're nocturnal and don't have any special lighting requirements.
as they're smaller as well, they don't need as much space and they don;t need a basking spot, you can give them a heat mat (which you aren't *forced* to use a thermostat with, but it's a good idea to, just in case)
you could easily get started with a leopard gecko for around £90, or less depending on what kind of viv you wanted to keep it in.

then there's snakes as an option, corn snakes, king snakes and milk snakes are all easy to keep, i don't believe that any of them have any special requirements for lighting, all are happy with heat mats.
feed those about once a week, and for all of them a 3ft viv would likely be fine for them when they're adult as well. (kings and corns can get up to 6ft long though, so if they did that, the minimum viv you'd need is a 4ft) , though with any reptile it's best to have the enclosure size increase as the animal grows, just so they don't get stressed.

starter kits for snakes and leopard geckos go for about £60 in the pet shop here, which isn't too bad for what you get. (also not a bad price considering the pet shop is run by a rip off merchant.)

other than reptiles, you could go for hamsters, gerbils, degus, rats, mice, a budgie if you and the neighbours didn't mind the noise. these are all quite cheap to house and keep.

you could even keep a house cat if you wanted in there.

then there is always fish as an option, but they can be hard to get right some times