If you just want to up the NPK (nitrogen, potassium, phosphorous) then Growmore is fine - the nutrients will disperse in the soil and are available for plants (it is recommended to rake the Growmore in the soil about 2 weeks before sowing/planting). BFB as mentioned is slow release because it has to break down in the soil first before plants can use the nutrients - when growing from seed direct, seedlings don't need fertilizer so the initial nutrient load will not be critical.
Depending on your soil type such as sandy soil, you may need "micro-nutrients" (calcium, iron, manganese, boron etc) which will not be in Growmore - if you dig in lots of compost and manure, these are locked in the plant material and become available as it rots down. Some sources are richer than others such as from seaweed, comfrey and nettle leaves which you can compost down or make tea and foliar feed instead.
Some plants need more of one nutrient such as calcium or magnesium regardless of soil type - crushed egg shells (or grind it to powder) in the compost can provide calcium, you may need to get some Epsom salts for magnesium when growing tomatoes and potatoes (all plants need magnesium but they seem to demand more)
For me, Growmore is cheap and provides a boost of the main nutrients especially after a wet winter before spring sowing and planting. I've had foxes on my plot in the past so this put me off using BFB and chicken manure but (fingers crossed) I think they've moved away. From an ecological point of view, a lot of commercial fertilizers use materials and energy which are not very green so ideally I'd like to shift to a more "organic" way of soil improvement.