Do you repeat what others state without thinking? 243 or any normal twist 6mm will be perfectly fine in the entirety of the UK for deer, except Scotland IF there is a legal requirement to shoot lead free. At present this is a game dealer association issue only.
The use of 80/85/90/95 gn lead free projectiles in 243 or 6mm will be more than adequate in Eng/Wal/NI.
That it should also be legal in Sco may well be addressed for the benefit of effective deer management.
If making suggestions, I'd rather err on the side of caution, knowing that the suggestions I give, are in no way derogatory to what the OP asks, BUT, may save hassle in the future !
Do you consider suggesting a 6.5, or bigger, poor advice ?