If you intend to create an open source project that could possibly be used across the globe, and if you wish to protect your trade mark by registering it, you will need to secure it on a per-territory basis. The WIPO Madrid system helps a lot in this matter nowadays, but US and EU only joined this system in 2003 and 2004, respectively. However, even today some countries require trade mark registration for their territory.
Therefore, it's not just a matter of a single 200 pounds for 10 years; you must multiply that by the number of independent territories on which you want to protect your trade mark. I expect that providing world-wide protection for a trade-mark was even most cumbersome than it is today when Python was created.
I'm only talking from my own experience, and I advise people to ask experts in that field if they need to secure a trade mark.