As the Armchair General stated, Irregular are the cheapest. But as the adage goes - you get what you pay for.
What are you looking for? A squad or two for skirmishing? Or hundreds to pour across No Man's Land en masse? If the former, I'd say spring for some nicer figures. If the latter, Irregular will get the job done.
As Mark said I have British figures from every manufacturer that makes then in 28mm. I just have the Irregular figures to do an I'll be posting some comparison pictures shortly (like in the next week or so) - stay tuned.
If you dig through the threads I did a comparison of all the Great War machine-guns a while back (sans Irregular).