Another thing to consider is:
Is your bankroll really $300?
If you lost that $300 are you just going to put another $300 online?
If so then you could say your bankroll is actually $600.
Obviously it may be likely that even if you could redeposit you might really not want to, but don"t get put off playing a bit higher if the worst that could happen is you just put more money in.
On the other hand - don"t be reckless just because you can put more money in.
There is another option that I"m sure some people adopt. They have a bankroll for a set period. So for example, they might have a bankroll of £500 for 12 months. If they use it all up, they might be able to start again with a bankroll of £500 for the following year.
This is probably a realistic way many recreational players approach the game. If a pro can have a bad month (or two) through variance - then a recreational player who plays far fewer tournaments or hands in cash games could go through the same variance, but it could be over a longer time period.
Personally, I had a much larger bankroll 12-18 months ago. I played a lot more online and had built my bankroll up considerably (from the pittance I started with). But I had to use a lot of this for "buying stuff" and so I have had to reduce the size of my bankroll available for poker and am trying to build it up again to where it was before. But as I"m playing less volume, and I"m playing at lower levels (because of my smaller roll), this is taking longer than I"d hoped. It"s always very tempting to play above your roll and take that shot. Fortunately, it"s something I"ve managed to avoid so far.