If you purchase something for the purpose of wearing it only for your group, you can claim the cost of the item against your taxable income for the year.
You can only claim for items which are wholly used for business purposes, such as uniforms.
Additionally, promotional materials (such an official item of clothing provided by Slimming World) can be claimed as it contains a business logo and would only ever be used in group or for promotional purposes.
Everyday clothing (even if worn for work purposes) cannot be claimed.