Hi
Alpharooms are based about 20 miles away from me in UK, used them a few times no problems as said above also look at Hotelopia they are owned by Tui, a German company but have strong connections with UK, they also own Thomsons one of our main tour operators.
I have used Hotelopia more this year as there are 10% discount codes around (was 15% in Jan) and we also get 10% cashback thru Quidco.com(doubt thats available to US residents but have a look) but no hesitations using Alpharooms either.
H/opia Downside is paying upfront but its only £15(about $30) fee to cancel, I just cancelled POR with them this past few weeks as Annual pass rate came out for AKL which was a similar rate to what I had paid for POR.
Alpha you only pay deposit but not sure about cancellation policy, never have.
If you are wanting to book with a big company then its H/opia, Tui is one of biggest travel Co's in the world. Alpha are still small but very reputable.