Do your research first to find out if there would be demand for this kind of restaurant. Talk to other vegetarian businesses in your area (B&Bs, healthfood stores, etc) to get ideas. Get in touch with vegetarian clubs and societies in the area and guage their opinion. The vegetarian Society can provide contact details if you dont know these.
Im a vegan and I live in Teesside, which is probably one the least veg-friendly regions in the UK. There is one vegetarian restautarant in the county and they've been going for many years and popular with both vegetarians and meat-eaters alike. Their secret ? Good food. That's it. If your product is really good, it will sell itself. They also diversify by having live entertainment during the week (music, comedy, etc)
Other considerations :Would you be taking over premises from an existing restaurant ? If so, you wont have the initial outlay of kitchen equipment.

YOGI_BRRRR - excellent answer. You pretty much said what I was going to say.

I think alot of this is going to depend upon the area. Some areas are friendly to such things, others, not so much.

However, I think if you put some kind of spin on the idea, it may help give you an edge.

For example: what about a spin like fresh, healthy and fast? There are so many fast food restaurants that will give heart disease, that something healthy could be a great alternative.

I'm just using this spin as an example, as it obviously depends upon your area. But the point is, a bit of creativity could work on your favor because people don't have to be vegetarian to eat in a vegetarian restaurant.

Good luck!
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Denise Taylor
http://answers.sitesell.com

I've seen quite a few vegetarian only restaurants around, and they have been very successful. However as the others have mentioned it really depends on your immediate area..4M in the UK doesn't help you much. Find out how many you have in your city!

It is very nice idea. I think , it is beneficial for your business.I see many Vegetarian only restaurant that have a big profit.Today many people like only vegetarian,so they choose only vegetarian restaurant.

Cheers for all your input and advice would like help if anyone thinks
They can add something to the business cheers.

This is a great idea especially with the argument that non-vegetarians will also eat vegetarian food. Keep in mind that this would be possible with the right push. A good tag line would be your best bet and since many non-vegetarians will likely pick your vegetarian restaurant for their first veggie banquet, you should offer something like a subtle conversion rite from omnivorous to vegetarian. This could be something like providing non-vegetarians a pamphlet explaining how well they have improved their system through eating a veggie banquet or something like that.