buying cologne cheap

Fragrancenet.com is a good website, quick delivery to the UK too. I believe all their stock to be genuine, although they sometimes sell Creed fragrances which are a tad old.

Personally, I'd pop down to Boots and pay the extra £20 so that you don't have to wait for delivery and you have definite peace of mind.

What fragrances are you buying, just out of interest?

I would reccommend to you Rochas Man, Play Intense, Code and Fuel for Life. Of course with £100 to spend you could go niche, have you ever thought about this?

I'd do a search on Google and limit it to the UK stores. You can also type into Google Compare then the fragrance name after it and it will being back all of the prices for you. Total them up with the delivery for what you want (ideally both from the same shop so you are ont paying 2 lots of delivery) and see if it works out cheaper than Boots. If there's not much of a saving in it, then pop into Boots
I've just had a quick glance and for a 100ml EDT Acqua di Gio it ranges from about £36 to over £50 online, so it could certainly pay to shop around.
Good luck

Edit: Boosh is spot on above with niche. Rather than the usual run of the mill counter fragrances that places such as Boots, Co-op etc stock, you could be adventourous and go for just one niche fragrance, or certainly two with £100 to spend. Niche is just an alternative to the mass multinational organisations.
From reading your other postings, it would appear that firstly you need to find out what type of fragrances you prefer the most and this isn't something you can find out until you've tried quite a few. Do you like woody, citrusy, aromatic, herbal, floral, chypre, oriental.... ?
There are many online sources where you can purchase samples from, so if this is your first time into fragrances then niche is out of the question until you work out what your favourite type(s) are, but if you've already tried and have fragrances, type in what they are that you like and people on here will be more than happy to point you in the right direction and recommend some for sampling that you may possibly like.

Less mainstream designer based fragrances, more purely perfume houses e.g Creed, Frederic Malle etc..

I prefer the summer versions of Le Male. If you have that option, it might be worth considering. Same Le Male experience, lighter, more modern feel.