Suffice to say, it looks like the minimum was set at an appropriate level in the legislation.
The legal minimum here for deer species on public land is .222.
Our smallest deer species is fallow. No fanged poodle sized deer here. The overwhelming majority of deer across the country are reds, which on average are as big or bigger than your Lowland variety.
The cartridge shooting by far the largest proportion of deer per annum is the .223 Remington. And 95%+ of those will be large reds.
Our forum has an ongoing thread on shooting deer with the .223 Remington that has nearly reached 500 posts. It started off as a question, then the usual armchairs said their bit, then those that are out there shooting deer regularly started posting photos with detailed analysis of wound channels, shot placement, bullet construction, weight, etc. The thread is full of the kind of information we seek in order to understand how to maximise terminal ballistics for this class of game, it has entirely shutdown the naysayers, and very clearly demonstrated just how lethal the 5.56mm is on medium game when setup correctly.
It is notable that many of the guys using .223 Rem for red deer are using traditional 55gr softpoints. Either their own reloads, or the factory ammo made by Belmont, our only domestic manufacturer. (Belmont uses once fired military range brass, their own powder blend and the classic 55gr Hornady softpoint that you buy by the 500.) Then there are the fast twist options and a variety of examples with some of our locally made bullets (Targex), TMKs, ELD-Ms, Barnes, GameKings, all sorts.
I mostly use 70gr Speer semi-points, but I also have a stash of the now discontinued 64gr Nosler “protected point” tactical type bonded bullets designed to shoot through walls to get to the bad guy. They are very, very effective on big red deer and will easily shoot through both shoulders of a rutting stag and leave a trail of carnage between them.
But… rules are rules. The Germans managed to convince themselves they need a 6.5mm to shoot a deer. It is what it is. They also like shooting at animals running at high speed, they’re happy to do that all day long! There’s no logic to it, really.