I seem to be unable to see why you'd want J1 visa. First of all, hour building could be done on visa waiver. Secondly, you'd have to show funding for J1, do the tedious paperwork and then you're likely to fly lots of dual due to school's policy and let's face it, more profit into their coffers as the instructors get only part of the hourly instructor's rate. J1 is meant for people coming over to do training up to CFI/FI and then instruct. If you don't need J1, don't do it as it's taking place someone else really needs.
Sure, you can find helicopter schools that could do a deal with you for some big chunk of hours. Then, there's the thing with risk of paying loads of dosh upfront, ie should be paid be credit card,escrow or some kind of deal paying along or what.
Sure you can have good time doing fun XC flights in the US.
As the guys count here, it's rather for guys living in the UK, namely rather England, for cheaper block hourbuilding. You will have to pay for living, landing fees etc, so..
Check out if that 210 dollars is all in. Even Bristow Academy started doing base rate, instructor and
fuel surcharge. Therefore dual in both J1 heli schools is around 270-290ph all in, if doing 'JAA programme', then touch more in Bristows. Again, Bristows combined JAA/FAA isn't really what you need, so no point JAA 'rate' towards CPL or hour building.
AFAIK, isn't UK CAA requirement 250TT in helis to start JAA FI? So you'd be able to do JAA FI in Bristow Academy in Florida (on M1 visa)after your hourbuilding deal (check out their start dates and availability well in advance and plan accordingly).
You can't change visa waiver to student visa, but you can get regular B1/2 visa for your hourbuilding (say you need more than 90 days for your hols and it'll be fine) and mention the possibility to change to M1 (going to check out school in person whether you like it, as a reason). You then buy yourself time, may or may not do the change of status, depending on need.
If you have enough dosh and your plan is to do JAA FI, having done ATPL (H) theory beforehand, you might be able to sort something out regarding CPL skills test with Bristow Academy before starting their JAA FI. Not sure if the exemption of having CPL for FI rating (with ATPL theory credits) is only UK CAA thing or the same all around JAA land. It's discussed in FW instructing in the UK all the time, probably just UK thing. After UK CAA FI course in BA you'd have almost 300 hours anyway if that's what you need to secure instructing job. Plus you'd have the rating already.
All this probably sounds too complicated anyway.
People who advocate hour building in the UK usually point out travel and living cost. Unless you have really good rate (airplane non-equity syndicate, not sure which heli school/owner would do block hourbuilding, I don't see into this one) and live in the UK, you'd still pay for living here. Plus if you can't get rid of UK VAT.. Hmm.
Don't forget weather. Helis have it better, but still.