That's good for you to be happy with them. I have to say that from my experience of wearing glasses since the age of 11 years I have tried everything and my opinion is offered from the 67 years of trying them all!! I do have a very complicated prescription which seems to get more and more so as the years tick on and this is where the lens offered through an optician wins hands down. The strength of the lens is the minor consideration, it then goes into the quality and subsequently weight and thinness of the lens that comes into consideration. Specs direct do not offer these specialist lens and if they did then you would find the cost would be vastly different!! Varifocal ( a revelation for me when they first came out!) is just a coverall name for a prescription lens that goes in channels from near to long sight, it is the optician that measures the exact requirement for each eye in order to get the 'centres' and channels accurately fit to suit the wearer. It does not surprise me that you were quoted £900 for the lens. These days, in retirement, I choose to go for the lighter, thinner lens in the second tier down as they are acceptable for comfort and vision and they come out at only £600 - £700 including frame and fitting.
PS. It was a revelation to me that trees outside my garden had leaves on them and were not just green 'blobs'! I can still remember the excitement of trying on my first pair of specs!