Why has the price for this game been so volatile, resulting in SIX price increases in the last year?
£7.19, £7.39, £7.59, £7.99, £8.39

GOG regional prices maybe? Asi understand GOG use USD for all regions and convert by own currency rate. So one factor:

- regional-but-still-in-USD price (set by publisher);
- second factor - no regional USD price (ones set by publisher who not always do that or use 1USD=1EURO, that some time ago GOG correct decreasing own % but that program of fair price is over).

Borh converted to local currency. But for one and for another there is differ changing rate when GOG do update their rates.
Current game base price: 10 USD.
http://www.gog.com/forum/general/local_prices_update/post676

And as i mention, about two years ago there is 'fair price' program in GOG that is over. US-USD is a main region and main currency. GOG actually use USD. And if thre is not set or by any else reason - any region price is higher in USD that US price = GOG will decrease publisher price to ake it lower than US, compensate that from own 'pocket'.
Now thay stop to do that.

Although for some regions looks like it never worked (Ru\CIS), but that how it be for UK and EU that always have it highest prices than US (or no regional price).