The idea of promoting the website on T-shirts is great although as mentioned they can be expensive. Perhaps a logo or the website address can be printed cheaply on to iron-on stickers. I'm going to order one of those tops I designed and then just wear it anytime I'm doing something aviation related.
As for the site name change, personally the change would be a good idea, just so that others know what the site is about in the name, but you have put a lot of effort into DalesPlanes so I can understand it if you'd rather not change.
What do others think?
By the way, NIaviation.co.uk sounds good. Would it cost much to change the name?