Wildflowers thrive only in the absence of vigorous grasses so if you have the machinery or access to it plough the lot up then seed with those bulk seeds mentioned. A mix of annuals and perennials will give you that first flush of blossom followed by a longer lasting flowers in years to come. Remember to include flowers that pollinators other than honey bees can use.
Failing that plant yellow rattle. It parasitises grass and will reduce your sward in the long term, then you can let your pasture return to wildflower.