It is a very broad question, i dont know all the good independants in the UK

infact there are some nice chains which will take independant clothing.
Without knowing the shirts you sell i can only give broad recommendations, in MK there are very few independants as the rent is too high in the main shopping area, the ones that spring to mind are things like skate shops etc.
If you go to a place like brighton on the other hand you will find a lot of independants.