I got some fleece cloth to protect my plants from frost / low temperatures, but it's semi opaque so blocks most sunlight. Is it OK to leave it on through the day (while it's still cold), or do I need to remove it during the day to give the plants enough sunlight?
You should remove it during the day; unfortunately, UK temperatures tomorrow are set to drop but hopefully not to below 5degC during the day where you are; unless you're growing something really tender, it's best to allow daylight to get to the plants. Replace the cloche after 3 or 4 pm for the time being until the weather's warmer, then you can replace it at dusk if temperatures are low enough to be a problem over night.
If it's very cold tomorrow, you may have to wait another day or two for temperatures to rise a bit; the other option is to replace the fleece with thick, clear plastic instead, which will keep warmth in and allow light through, although it will get too hot inside in full sun between 11-3pm, so will need to be removed in that situation.