It would be helpful to give a good description of the type of soil that you are dealing with including soil ph for an answer.

Agree with Loosestrife. When you dug out the planting areas what did you actually do? Remove the turf, dig over the soil to at least a spades depth and add organic matter?

You'd need to incorporate humus rich material, and that means things like composted animal manures, well composted leaf mould, spent mushroom compost, your own good garden compost, anything like that you can get hold of.

As you haven't said what's wrong with the soil, adding humus as above you can't go wrong. However if the ground is very heavy and sticky adding some coarse grit would help. And try doing a soil test - you can get the kits quite cheaply from any garden centre. They will show you which nutrients are lacking, if any, so you will know if any lime or fertilisers are needed.

I would get a few bags of Horticultural Grit Sand, if you are only dealing with the beds in your lawn, its marvellous stuff for improving clay soil. Easy to carry £1.99. Also look around for a stables/riding school where you live, they are only too glad to have the manure taken away. Stack it for
a year if growing vegetables, if only the flowers you mention it can go in straight away, with some granulated
Fish, Blood and Bone £4.99. You will be pleased with the results. Give it a good stir!

A few cubic yards of fresh topsoil would help as well. Always be sure any manure you use is rotted for at least 6 months. Never use fresh manure in your garden - it has too much salt and ammonia which will burn your plants.

Definitely get one of those little test kits from a garden centre - find out if your soil is overly acid or alkaline. From the plants you described it could be either! But if it is particularly one way - you need to correct it - or only grow plants that enjoy that soil.
Compost always helps - but again - after you know your soils pH - mushroom compost for example is quite alkaline - so if your soil was already alkaline you would not use that. Mulch your plants when you do eventually plant up - it keeps the moisture in and brings the worms to the surface - and they are the ones that make your soil good... !