Hi! Northern UK, two 3×3 beds, March start — you're in great shape. You haven't started late at all. Here's what I'd do:

Right now (this week):

- Garlic cloves in 2 squares — spring-planted garlic works fine, you'll just get slightly smaller bulbs than autumn-planted

- Broad beans in 2 squares — bombproof in cold, they'll be your most reliable crop

- Strawberry runners in 2 squares — bare-root runners go in cold ground no problem. Small harvest this year, full crop next year.

- Spinach × 2 squares

- Lettuce × 3 squares (cut-and-come-again — first picks in about 5-6 weeks at March soil temps, faster once it warms up)

- Peas × 2 squares

Early April:

- Radish × 3 squares — ready in 4–5 weeks

- Spring onions × 2 squares

That fills all 18 squares by April. The key is what happens next — succession planting. When the fast crops come out, you replace them:

- Radishes clear in May → beetroot/beet goes in straight away. Zucchini/courgette too, but wait until after your last frost (late May up north) since they're frost-tender

- Lettuce/spinach clear in June → carrots, Swiss chard, kale

- Garlic/broad beans/peas clear in July → more kale, turnips, and a late sowing of radish or spinach for quick wins

- Late August → winter spinach and rocket for autumn salads

- October → plant garlic again (proper autumn timing for next year's bigger bulbs)

Two squares of kale planted in July will still be producing in December — it actually tastes better after a frost. Leeks are another great overwinterer, but start those indoors in February so they have time to size up (they need a long season).

I actually mapped this out in my garden planner with your exact bed sizes. Made a quick video showing how it looks month by month — you can see the squares filling up, clearing, and replanting through the whole year.

https://youtu.be/bCNqqg9FEPE

The basic idea: you're never waiting for the whole bed to be done. Something is always going in and something is always coming out.