I've had excellent service from Wiggle historically, so no need to criticise there, but I must give a big thumbs up to Merlin.
I bought my Turner Flux from them in an order and discussion process which stretched out over a good couple of weeks. I took their standard build option and completely pulled it apart, generating a totally custom spec with each component chosen and discussed (right down to evaluating nipple colour and material for the wheels, rotor sizes and spec, quick-release choices and pre-adjusting travel on the forks).
Depsite all the extra hassle and time spent, they delivered everything spot on and in terms of cost, they allowed me to calculate everything using their website with adjustments up and down based on their published spec.
I know this is perhaps to be expected, but I've rarely had such good attention and service from a LBS, so to have it from an internet retailer (okay, so they are a bike shop too), was a very pleasant surprise.
Based upon my experiences to date, I would have no hesitation in going for a fully-custom build with them again.