You don't need conditions to be crystal clear, often a little murk helps hide you from the fishes. However, you do need to see beyond the end of your spear, ideally a metre or two or more.
Flat(-ish) seas, low wind and/or wind blowing from shore-to-sea seems to help. No rough seas/storm conditions/heavy rain for 2, 3 or 4 days before hand. That suggests a lot of days in the UK will not be suitable for spearing - which is true - but some areas are more vulnerable to , say, rain or wind from certain directions than others. Bays, harbours, headlands, etc. often have their own micro-climates that are not reflected in BBC weather forecasts. Also, even with the above conditions favourable, you might still encounter poor conditions due to, say, "spring bloom".
So my suggestion would be to avoid obviously adverse conditions but do not wait for perfect conditions, they can be few and far between. If you encounter poor conditions, learn what you can from them.