Be very careful buying overseas and be aware of all the costs and issues.
There are a few people here (in this forum) that have gotten ripped off bu UK sites. Do beware.
Also being sized by a website is just not going to work. Many people have done with bad results. People seem to forget we are all different and there is no way a formula on a website will give us our perfect bike.
I sell bikes online and have for years and see no problem in it. People will still be scared and I do not blame them as many sites out there have no clue on cycling. So people tend to fall for those due to low prices.
Bottom line is you need to know what size frame you need (top tube lenght). Do not let anyone determine this for you blindly. They would just be guessing.
Too many people call me and say I want a bike to fit a 5'8" guy. Impossible for me to sell you that. But some shops do. That is scary.
Do they think everyone 5'8" rides the same exact frame worldwide? NO!
Just know your top tube lenght and any other measurement you need. With this info you can buy a bike anywhere and everywhere.
I wouldn't buy a bike online, unless I knew the exact make, model and size I want. Even if online sites offer a fabulous deal, there is no substitute for trying it out before you buy it.
If you dislike the bike or have problems with it, returns will be more of a hassle, even if the company offers a great policy. Packing it up and shipping it is more difficult than tossing it on the back of a car and bringing it to a shop.
Get advice; get parts; get accessories; but buy the whole big in a shop.