It's a tricky one. I don't think the Consumer Contracts Regs are that clear. They specify seals in relation to CDs, DVDs, Software and items specifically made to order, but I don't think they envisaged security seals on stuff that doesn't need security seals.
I think I would speak to Citizen's Advice as it may be considered an ‘unfair contract term’, although I think you may not be on solid ground here. Their useful contact page is...
Contact the consumer helpline - Citizens Advice
It does seem that trying archery products, to check they are correct for you, is becoming a thing of the past.