Great Plains is a true-to-size brand. We'd recommend buying your true size in Great Plains. All sizes on the Shirt Sleeves website are in standard UK sizing.
UK Standard (2-3 days): £3.99 (Free over £50)
UK Next Day (Next day): £5.99
Collect in Store (Next day): No Charge
European (3-5 days): £6.95
US & Canada (7-10 days): £10
Rest of the World (7-10 days): £10