BRP is definitely missing the details of an encumbrance system. So I came up with the following (which, I hope, doesn't too badly contravene what The Book says):
Encumbrance Capacity: SIZ x (STR + 10) / 20 (x14 lbs). Over half Encumbers: Movement, Initiative & Dex-based skills are x1/2.
Armour Burden (Light/Moderate/Heavy) can affect physical (generally DEX-based) skills, movement rate and magic spell-casting.
Light: no skill penalty, -1 Move, +10% Magic fumble. Moderate: Skills x1/2, -2 Move, +20% Magic fumble. Heavy: Skills x 1/10, -3 Move, +30% Magic fumble. Listen/Spot skills are greatly disadvantaged in Full/Great Helm; Lock-picking, Shooting Bows and Playing Instruments are impossible in Gauntlets. Anyone not trained to the burden of armour is automatically Encumbered.
Fatigue Exertion (e.g. combat) for a turn requires a CONx5 roll to avoid Fatigue (Movement, Initiative & all skills x1/2) or if already fatigued Exhaustion (x1/10). Recovery from fatigue requires rest for 1 hour; exhaustion 8 hours.