@Dave7 I fully agree with
@Paulus . My working life was spent in horticulture, much of it working with garden centres. A couple of other comments.
Pansy can get frosted and the leaves turn black. This would happen at around -6/7C. Thinking about Geranium the traditional date for planting more tender plants is May 15th onwards. This is a median date and can vary by 2-3 weeks either way depending on location.
If by adding compost you mean something you buy in a garden centre my personal view is this has little impact. A peat based compost can/will dry out and won't aid moisture. If you use a non peat based compost water retention is better. If using peat a cheap tomato growbag will do and is probably better for this job.
Regarding garden centres not covering geraniums. It's possible the construction of the canopy stops frost coming through. It's quite likely the centre will cover with fleece at closing time.
On the question of frost. I had 22 years in bedding plants. If the forecast was +4/5C we would protect against frost. It's easy for temperatures to hit 0C and lower around 5/6.00am. We grew millions of plants and were rarely caught out using this rule.