My experience is that for £150-200 - which is what my punts average out to - you get independent escorts with consistency, reliability, and a degree of availability that you don't get at £100.
Above the £150-200ph mark you get into "elite escort" and "agency escort" territory which is a completely different market. Here you can pay much more for the same service (or not as good) - agencies in particular seem to rely on a more affluent clientèle maybe businessmen or travellers who don't have the time to research independents or are unfamiliar with the London escorting scene and as a result are willing to pay extra for "convenience" vs quality of service.
For independent escorts it's not rocket science - a decent escort might start at £100 ph (I know many top £200ph escorts today started at £100ph), but if they deliver consistent quality service guys will come back and word of mouth will develop. Some of the very best escorts as per reviews on this site started out exactly like this.
At £100ph such a WG will then get VERY busy and difficult to book, so either she burns out trying to fit in 10-15 bookings per day and as a result something has to give - either quality or availability suffers. The only alternative is she ups her rates to a point where the bookings become manageable again. Doing so she inevitably finds she makes almost as much money with half as many bookings. She's happy, her regs are still happy, and she'll get new regs who are also willing to pay that bit extra for the combination of reliability, consistency and availability.
Only those unwilling/unable to see her at the new higher price lose out.
If she's not good enough to warrant a higher price, her clients will drop off and she'll have a dusty phone until she gets to a price point that's appropriate again.
At £100ph you do occasionally find a gem - but in which case they will either be new and therefore a risky punt, or else they will be inconsistent, unreliable or not readily available. Despite protestations to the contrary I have still yet to hear of any WG in London / surrounds who charges £100ph and provides a consistent, reliable and good standard of service while being reasonably available to book. At least not one that delivers the services I need for a decent punt. There always seems to be a compromise somewhere. At £100ph if you're good you're fully booked, if you're readily available it's because you're not good! IMHO.
For £100 you pays' your money you take your chances. Ce'st la vie.