B&Q in the UK

They offer a good range of ropes, any ideas on suitable thicknesses?

Go to any good marine store.

They will have the best selection anywhere of ropes colors, diameters, braid designs and textures.

I agree yacht chandlers and Caravaning stores stock a variety of ropes in various thickness and colours. However this tends to be modern nylon rope rather than traditional rope which I am sure would feel far more authentic.