Londonlottie, that's an excellent way of looking at it - as a course of treatments which you continue until you get the baby, rather than looking at it per cycle. Incidentally, London Women's Clinic offer a 3 for 2 deal on IVF! You pay the lot up front and it covers you for 3 treatments!
Here's my major caveat - I think the approach of starting at a 'less successful' clinic is a good one ONLY if you've got time on your side, or maybe if you're entirely male factor. If you're only just starting out and the wrong side of the magical 37, then I personally think you've got to bring in the big guns straight away.
I found this illustration really useful:
www.in-gender.com/XYU/ART/Cumulative-Odds.aspx
Using the figures on here as a rough and ready reckoner, you'd need at least one more cycle, you'd need three times as many cycles at Guys than you would at ARGC to hit the same cumulative rate. (I'm basing that on a comparison of the live birth results reported for Guys and ARGC for 40 year olds having ICSI, which is 11% and 27% respectively.) So, rounding the figures to the nearest marker on the graph, you'd need 6 cycles at Guys to get to a cumulative success rate of 47%, and only two at ARGC to get to a comparable rate of 44%. So at £4k at Guys, that's £24k, whereas at £8k at ARGC, that's only £16k.
I poured over these like the nerdy statto I am, when we were debating where to go, and when time isn't on your side, the extra few percent can make all the difference!