courgette

Why is the leafs yellow

Get a bottle of Maxicrop and give them a dilute feed because it looks like they are deficient in some nutrients.

Would tomato feed  be okay ?

Not really the same thing, although it wouldn't do any harm. The whole point of sea weed is the micro-nutrients, and to treat deficiencies. Tomato feed is all about the macro-nutrients (N/P/K), and specifically the K, which promotes flowering and fruiting.

By the way it’s called a gold rush courgette

That is a yellow variety, so the leaves will yellow naturally in most cases in my experience, even when it is well fed, and without any deficiencies. Assuming you prepared the soil well (lots of N) your courgette should do fine without any extra feed, at least in the early stages, but courgettes are very heavy feeders, and may need a boost further down the line when the nutrients in the soil start to get depleted. 

Personally I give them dilute fish mix (Biobizz) + seaweed (Maxicrop original) when I think they need it. Great combo, and can be used on everything else pretty much, also with the advantage of being totally organic.

Bottled chemical feeds like tomato feed will likely harm the soil micro-fauna, which is one of the things that really helps plants cope (with everything from pests, to drought), so by using organic feed I'm raising plants that will better be able to fend for themselves without being mollycoddled. Feed the soil, not the plant... and the soil in turn, feeds the plant.