I know it's hard. However, it really is something you should've thought of beforehand.
Apologies but I don't think Milk was saying your prices are too low either. It was a combination of price and design she mentioned. I agree based on some of the designs,specifically the BBT one which is done by everyone. EBay would get it out there with the rest more and is probably the right market. That or Amazon.
Where did you get the ones you currently have printed? How much did that cost?
The thing is with anything someone designs and makes is that they also need to factor production in first. Some things can be made to order,some don't sell so well if they are. But what I'm wondering is how you were planning to supply any customers currently if UK printers charge so much?
Have you asked about volume printing costs and business prices?
I've just googled and found www.screenprinting.co.uk 10 t-shirts for £4.79 each.. Yorkshiretee offer a printing and drop shipping service.
But as I said, and it's not about 'my way' it's about getting a business head on and doing it the right way, it is really something you should've looked into first. What would've happened if you had got loads of orders?
Also, honestly, really don't sell you designs for people to print themselves at £3. Seriously. You won't do yourself any favours and they could well wonder if you are printing them out and ironing them (onto primark bodysuits), as you've said they can. It lessens your chance of an actual product sale.
As for not getting many viewers. You have to work at test pretty much daily,at least to start with. Most etsy sellers moan and most say how much work it is to get those viewers. You have competition to battle with.
You also do need to check out the competition 
And I know it's hard. I'm a single mum with a severe and complex autistic to look after. I run my own business as a jewellery designer, goldsmith and silversmith. I started with an investment of less than £100 on silver and gemstones, made one of each of the initial collection and put them online as made to order so payment was received before I paid out for more materials to make more. Now I can also produce more before sale. I design,plan,scale, hand cut,forge,set stones and polish, photograph,run the website and marketing,package and post everything myself and it is a lot to do. BUT I can do it because I planned before I started,including making a marketing plan. The initial planning really will it much easier and more viable for you.
Get yourself a social media business page too.
Update etsy as much as possible. Get networking etc. Consider other selling platforms as you need to find what is most suitable for your products.
What quantities have you been looking at getting printed currently?