In my experience I have found the best calibre for me to be the 6.5x55. I have had a .243, 25-06, .270, .223 and a 6.5.
The .270 did drop beasts on the spot, but it was very damaging on the carcass and not the nicest calibre to shoot.
I found that the 25-06 dropped roe efficiently beyond 140 yards with a gameking 117gr. Under this distance and certainly below 100 yards, deer could run on quite easily.
.243, roe deer can run on with 100gr bullet, with a 80gr I found that they dropped quicker. I wouldn't use it on reds in the woods, but would on the hill, but expect them to run on unless you are neck shooting. Shot a red in the rut in the engine room and it was running for about 80 yards, shot it again in the same area, opposite side. It ran on another 50 yards and went down. Got to it and it was still pi55ed off. Coupe de grace. Opened him up and the lungs were in bits and there was no heart. 2 X perfect shots. It was enough to make me never use a .243 on a stag again.
.223, yes they drop deer, but I wouldn't use one again. Just personal choice.
6.5, Every roe I have shot has dropped on the spot. Every red has dropped within 5 yards of impact except for a hind that was watchig me for about 20 seconds before I shot her. She ran between 15 and 20 yards whilst full of adrenaline. I have also used the .308 but for me the 6.5 is the most comfortable hard hitting round about. It is inherintly (if you spell it that way)

accurate. Also, if it is that bad a calibre why have so many elk and moose been taken!
To answer the original question. If you were going to shoot most species of deer in the UK, then the 6.5 is the answer. There has also been comment on the trajectory...... My 6.5 shoots 1.3" high at 100yds. At 170yds it is spot on and at 240 to 250 yds it is 6" low. If you cant shoot a deer in this country at these ranges, then you should give up.
J