I started cycling seriously back in the early 70's, and the local shop where I bought my first good bike also sold Bob Jacksons. However, I have never been particularly impressed by British framebuilders. During the bike boom of the 70's, all sorts of British bikes made their way to North American shops, many of them having rather questionable workmanship. Not saying that Jackson or Mercian were among them, but one telling feature was the use of a 26.8 seatpost. A Reynolds 531 seat tube is single-butted, and the wider inside diameter (27.2) is supposed to be at the top. So if your frame (almost always English) required a 26.8 seatpost, it meant that the builder had brazed in the seat tube upside down! This was fairly common on the British frames.
Unless you just want the British name adorning the frame, I think you are far better off having a US builder construct your frame. Besides the money staying here, I really think US builders have outstepped the constraints of "traditionalism" in frame building, being not afraid to use state of the art tubes and welding techniques, depending less on "we've always done it this way." At least, that would be my assessment, and I'm Canadian (and can only think of a couple of builders in Canada I would trust, not including Marinoni since that crash on the track back in 1980 caused by someone's Marinoni fork snapping!). Plus, as mentioned, warranty issues are easier dealt with.
Living near Seattle, I've been riding the heck out of a Rodriguez street fixie (built by Dennis Bushnell) for the past 3.5 years and 75,000 km (including PBP, LEL, and a CA Triple Crown, and soon to add a Furnace Creek 508), and I am really impressed by its durability (and its geometry, which allows me to make stupid mistakes when I'm tired, and it stays up!). Anyway, I think you're far better off with a frame made by a reputable US builder. One thing for sure, it's not Mr. Jackson or Mr. Mercian building that English frame anymore. Which could be a good thing, or not.
Luis