We tend to hold on to "I'm a UK9" and struggle to deviate from the shoe size we were and that we think we still are - in our everyday shoes and activity footwear. 
There are fit and sizing differences between brands and even between shoe models within brands. Just because you have 'always' been a UK9, it doesn't mean that you are always a UK9 over time and across shoe models and brands.
Feet do change size as the muscles in your feet develop with use. Your foot size and shape will change with type and volume of activity, and age. 
What happens when shoes are too tight or too small? Ill-fitting everyday shoes can result in physical deformities. In our activity shoes, too small or too tight trail shoes and hiking boots will deliver blisters and toenail damage, pain and discomfort. Because of this, you may start limping and walking unnaturally. This in turn will alter your alignment and posture, and could result in back, hip and knee pain.
 
What we do not always consider is the effect that our everyday shoes have on our feet. Heels, tapered and pointed shoes and too small shoes alter the shape of the foot and its underlying bone structure. Hammer toe, crossed-over toes, bunions, and corns are common problems that can be corrected with simple measures if caught early, but later may require surgical correction. These foot issues can cause problems when walking, running and hiking, and require even more consideration when choosing shoes for these activities.
Do not be unwavering about brand, model or colour when shoe shopping. Try everything suited to your activity, keep an open mind about the sizing number and buy the shoe that fits properly and is the most comfortable.
AR Gaiters go a long way to reduce the risk of blisters and discomfort by keeping grit, seeds, sticks and stones out of your shoes. With AR Gaiters and properly fitting shoes, you will be set for success.

 

TOP TIPS
  • ALWAYS take your own socks along to try on walking, running and hiking shoes.
  • Try shoes that are slightly bigger or smaller than what you think you are to fully gauge and compare sizing and comfort. Keeping a note of models and sizes together with a rating i.e. 2/5 or 4/5 will be advantageous.
  • Make sure that your toes are not squished, pinched or cramped (on the sides and front), and that no toe crosses over another (pay attention to the baby toes too).
  • With modern materials and construction, trail shoes and hiking boots do not need to be worn in to be comfortable. They should feel fabulous when you first try them on.
  • Check the flexibility and rigidity of the shoe. It should bend in the toe box area. Forefoot protection plates sound beneficial but they can prevent the toe box from bending and will limit mobility of the shoe.
  • Assess the height of the heel counter. Is it shallow or high? If high, will it jab into your Achilles tendon when going down steep descents?
  • Have you got foot conditions? Bunions, corns, hammer toes (scrunched toes), Morton's Toe (longer second toe), crossed-over toes, an arthritic joint or a wide forefoot. A squared toe box, or as wide and rounded as possible, is the best shape for you. A wider, rounded toe box will make space for your toes to spread as you walk and run.
  • Does the depth/height of the toe box accommodate your toes without pressure on top? You need around 10mm clearance above your toes.
  • Always choose shoes with removable inner soles. If your feet are swollen or you realise that you bought too-small shoes, the inner sole can be taken out to give your feet more space.
  • Ensure that there is some space in front of your toes (1/2 thumb width) and to the sides of your big and small toes. 
  • Any lumps or bumps that prod into your arch will not go away with use. That shoe is not the right shoe for you. Mid-foot (and heel) should be snug but not tight and constricting.
  • Different lacing patterns can improve functionality of the shoe, like by using the heel lock lacing or skipping a cross over across the bridge of your foot to reduce pressure there. Check that the laces are long enough to do the lacing pattern that you prefer.
Bad feet images: from the internet