Hi Alz, having gone through this process myself (I'm a UK citizen and my husband is from China), all I can say is that you shouldn't do it. It was a horrible, horrible experience for us that nearly broke our relationship. We were treated like criminals from start to finished, paid thousands and thousands of pounds and waited for many months without any information and without my husband having the ability to work or leave the country while the UKBA 'considered' our very straightforward application. And it wasn't just us - it was hundreds of other married UK/non-EU spouses in the same situation. I can't begin to tell you what an awful period in our marriage this was.
In all honesty, you would be incredibly naive to think that this is going to be an easy process because you have a UK passport. This is particularly so as you're not married to your girlfriend. Currently, the UK government is following a 'hostile environment' policy to discourage immigration and this really hit us hard when we applied. With Brexit happening, I think it will be even worse. If you're going to do it, please prepare yourself for an extremely bumpy ride.
Another point is that why would you want to go to the UK at all? My husband and I left the UK a couple of years ago partly due to the horrendous treatment we received at the hands of the government. But we also left because the UK is just a miserable place at the moment. The weather is grim, the food grimmer, people can be racist/hostile to immigrants, and Brexit is coming so Brits are just as grumpy as Hong Kongers. It's difficult to save because you get taxed at a very high rate and the housing is extremely expensive (though admittedly not at HK levels). Commuting in places like London and Bristol is rough. Anti-social behaviour and street harassment is a problem in the UK in a way that just doesn't exist here. I know that life is not always ideal in HK but seriously, the grass is not greener on the UK side.
My advice would be to stay put in Hong Kong - besides the high property prices and lack of space, it has a huge amount to offer for the time being. Invest in property in the UK if you want to make the most out of that market, but wild horses wouldn't drag me back to live there.