First off, why would you want to invest your money in the UK?
All joking aside, it is possible to invest in the UK via Hong Kong.
Most companies have what are called ADR (American Depository Receipts) in the USA. It allows for non-USA companies' stock to be traded in the USA. I believe that all major exchanges have a similar policy for cross-listings.
So, you could invest in BP, Diageo, Unilever, or whatever, via Hong Kong. The price of the stock will be affected by the exchange rate, but it will be a wash transaction. For example, if BP trades in London at 30 GBP/share and the exchange rate to HKD is 10 to 1, then you'd have a pricing of 300 HKD/share. If the HKD tanks, then the price of the share would go up by a corresponding amount. Thus, you'd still have 30 GBP/share value. Note: ADR's are priced differently than their original listing, so the simplified example above will not correspond to real life examples.
This is how Americans can invest in Daimler Benz, BMW, VW, Royal Dutch Shell, BP, etc. via the comfort and ease of the NYSE.
TL;DR: Buy your UK stocks on the Hong Kong exchange and sleep well at night knowing that your money is invested in the UK, not HK.