You can get railway sleepers and half length sleepers from Wickes. They are very heavy but they will deliver.
Some herbs are Mediterranean and love the heat but are perennials so come back every year and they like soil with very good drainage eg thyme, rosemary, sage - they will grow well as long as they do not have wet roots. These often do not lose their leaves and you can use them all year round.
Some like good hearty soil, eg chives, oregano, and will also come back but aren't too fussy about the dry soil aspect. Oregano makes a good flower for the bees but be aware if you let it set seed it will seed itself everywhere. Ditto Marjoram which is Oregano but just a slight difference in taste.
Some like mint, are actually water lovers, but will send out shoots underground and try to take over whatever patch you give them, so are best grown either in beds of their own, or plant the mint in a large pot, with holes in, and bury the whole pot in the soil to contain the roots if you want it in a mixed bed.
Some like parsley, including the flat leaved type, are biennials so will try to flower in year two but will stay in leaf all winter long so I always sow a new batch in the spring leaving lots in the garden each winter.
Basil, Coriander and Dill, are hot weather herbs and quite difficult to grow from seed. During the 7 years of teaching organic horticulture, although I did grow quite a few good crops, my best ones were to buy a basil plant at the supermarket, and take cuttings and plant those out after the last frost date. Dill will bolt like crazy so you have to keep chopping it back. Same with Coriander, but there are two types - coriander is grown for the seeds but cilantro is grown for the leaves so if you want it to go to seed slightly less, buy Cilantro or Leaf coriander seeds or plants. Again I have kept coriander going from supermarket plants for months, much easier than growing from seed.
I would like to offer up as well, getting some Garlic chives; these are flat chives, which taste of garlic. Which means you can get a good garlic taste for cooking; without having to wait each year for your garlic crop which can be hit and miss.
Also, Lemon Verbena, which is a perennial [it does lose it's leaves in winter] and has the most divine scented leaves. Chamomile is a great short herb which can go under the taller sages or rosemary. And Fennel - you have to have a fennel just for the glorious scent and you can leave it to flower for the bees and use the seeds in cooking.
Also to make use of the space, as the annual herbs die off each year, fill the gaps with some leeks which will grow all winter, and be harvested before you put next year's tender herbs out.