I'm sure it'll be a fine substitute, providing the SRM isn't too high. I use Gleneagles Maris Otter as my house base malt---the Gleneagles has an SRM around 3.5-4.5, a little higher than normal Maris Otter---but I like it that way---I think it's got more flavor than the normal MO. Anyway, check on the SRM just to be sure, but otherwise you should be just fine.
As to the percentage, that depends wholly on the style of beer you're brewing. Since it's a well-modified pale malt rather than an adjunct grain, you could use it for up to 100% of your grain bill if you wanted to, but again, it depends on the style. Personally, I've never used 100% of anything. My base malt makes up most of the bill, then I use a smaller percentage of adjuncts depending on the recipe.