Hi 38 degrees have a seeds for bees mix, but they do ask for a small donation to cover postage costs.
Innocent this year are sending a pea kit out.
Wilko and poundstretcher do a big box of wildflower mix for about £3 and once established you can harvest the seeds at the end of the summer for the following year. Or some of the annuals are self seeding and come back year after year.
Lidl also do packs of seeds 5 for £1 and not wildflowers but can get sone that are beneficial for bees.