Everybody has their own opinions, but anyway, here are mine
1) I have never shopped for perfumes in London, but from previous threads (you can search) London has pretty much everything safe the Serge Lutens exclusives. In addition to Selfridges and Harrods, many perfume brands have their own stores (from designers like Chanel etc to niche like Ormonde Jayne, Amouage etc). Don't be intimidated by sales assistant. At the beginning, just smell. You'll buy later.
2) Perfume evaporates faster with heat, and the body is much warmer than paper. So perfumes on paper will evolve much more slowly and will keep the head notes longer. There could be exceptions, but usually, they smell the same, it just takes longer to go through the various phases on paper. As you say, paper is a necessity, so use it. The bigger problem is nose fatigue. After you smell 4 or 5 perfumes, your nose tends to shut down and you can smell much less. You have to time your smelling trips so as to leave some time inbetween for your nose to reset. And of course, you have to bring the paper strips home and smell them later at ease.
3) No precise definition of niche. Usually, niche refers to newer perfume houses that only do perfumes and have a restricted distribution (own boutiques or a small number of upscales stores). Examples are Serge Lutens, Dyptique, Amouage, Ormonde Jayne. It is usually constrasted with "designer", which usually refers to bigger firms who produce different types of luxury goods and have a larger distribution (eg Chanel, Prada, Dior). Both niche and designer are "commercial", of course.
4) There are great affordable perfumes. but many cost a lot. One way to get them is to buy samples (there are firms that sell small samples) or decants, or share the purchase with others. After all, you don't need dozens of bottles of perfume. And yes, there are a few great perfumes in that price range, if you buy the smallest bottles.
6) Most people in this forum don't believe that one should match fragrance to occasion or sex (with exceptions, of course). So just wear what you like.
7) I'd encourage you to smell extensively before committing to purchases. You need to develop your own preferences, and see what's around. You can also get samples, as mentioned above, sot that you can actually wear them. Anyway, people here have heterogeneous preferences, so if you ask what to buy first, you'll probably get 100 different answers. Still, no reason not to express my own preferences among less expensive perfumes. As said, if you get the smallest bottle (30ml or 50ml) you should be within the price range.
Eau Sauvage (fresh)
New York by Patricia de Nicolai (orange amber). The niche brand has a store in London and, unlike most other niche brands, they sell inexpensive 30ml bottles. Most of the stuff in their range is good. Other PdN suggestions include Vie de Chateau and Temps d'une fete.
Aramis (stronger, almost leather). Most of the Estee Lauder/Aramis range has a good quality/price ratio.
Breath of God by Lush (weird smoky).
Caron pour un homme (lavender vanilla).