If you are willing to consider aluminium then you can get some real bargains:
Mountain cycle rumble, Marin rocky ridge/b17 or a Saracen zen for under a hundred quid. You might get a cheap stiffee or chameleon for about 200.
If you are set on steel then as others have said, trailstar, 456 summer season are cheap and will do the job.
Personally I’d look at the less popular brands and try and pick up a bargain. That way you will have more to spend on forks and the rest of the build.
I got a really expensive frame for £110, due to it being a brand that is not trendy.