I thought 5 quid looked like squid. As I'm assuming most of your sales will be internet-based, I think others will read it that way too. A squid wearing a tee is an easy way to remember "5 quid tees" - so it becomes not only a mascot, but also a memory device. The squid alone could work as an excellent bug/small mark/icon for smaller imprints - perhaps the silhouette of the squid with a 5 in it.
Additionally - for international customers who aren't as familiar with the term "quid" in lieu of "pounds", it will help them to remember squid = 5quid.
its a nice idea and actually one of the name we were going to go with was 5 squid tees (Squid being slang for pound also), however we opted for 5 quid tees and ideally i'd prefer customer's to remember it as that, as i think the squid pulls away from the original main allure of the site, the fact that the t shirts are cheap but very designed.
Sounds good. For what it's worth, I have quite a bit of experience (at least in the US market - just google josh eacret) regarding tshirt design / online sites. I would recommend adding some sort of "hook" - cheap tshirts tend to be seen as lower quality regardless of the ACTUAL quality. Just my two cents.