we already jumped through the hoops back at the London American Embassy to get the tourist visa which don't forget also lasts for 10 years.
When I used to travel to the US in the 70's, the visa in my passport from the American embassy in London was "unlimited". That didn't give me the right to enter the country; It was still up to the guy at the first port of entry, and the "hoops" jumped through in London, albeit relatively minor at that time, were just part of the process. On arrival in the US, I'd get asked questions and answer politely, with a few "yes sir" and "no sir" thrown in for good measure. Admittedly, the questions were a little different back then, but the process is essentially unchanged today.
Terrorism aside, the officer at the border has to be satisfied that you don't plan to set up residence here, work here, or end up being a drain on our economy. Coming in for a second, third, etc 6-month term might suggest that your intentions are somewhat more than being a tourist, and the I-539 you filed (?) would merely add to the suspicion.
I've witnessed people at US embassies/consulates and at borders try to prove they planned to leave again, but their proof was usually little more than a return ticket. I also know what I went through to sponsor family members to come here, and what they went through on arrival. All valid IMO. Next time you fly back to the UK, check the non-UK/non-EU lines at immigration.
BTW if you go south at the end of 6 months, you can return the same day without answering any questions; Just hop over (or crawl under) the fence with the thousands of Mexicans who do it daily. If they catch you, all they do is send you back, and you can try again the following day. There's apparently no limit on the allowed number of attempts.