I'd opt fer the 20" blade.
I'm about 190cm and 11 stone
I'm a fan of short swords, and have a few meself. Most were designed for:
1. Use indoors (baselard, Wak) or in crowded conditions (Gladius, Falcata)
2. For persons of smaller stature
3. Backup weapon to main battle weapon
4. Easier concealed carry
5. to meet local weapons regulations on blade length (certain waks in Japan were katanas that were cut down to get under the restrictions)
Short swords and long knives were sometimes made from longer swords that broke.
You mention your limited knowledge of the use of swords-- have you used long blades before, such as a machete? Same safety rules apply. Seek training, if not available, be safe.
BTW, I thot guns were illegal to own in Britain..Wait..yer an Auzzie?
Keith