There is no formal dress code in the UK, unless you are mixing with Royalty. In my opinion Camouflage clothing is effective, many people wear Camo, and many wear plain dark olive.
If you are new to stalking, gloves and a face net are important concealment aids, a good hat and a jacket with a hood also helps. Make sure that you have plenty of room in the clothing for additional layers in the winter. Gortex boots for the summer, and neoprene lined rubber Wellington boots for the winter.