To be honest, this lawsuit as well as the help from a MP would do little in propelling Britain to do anything on Hong Kong.
The fact is, a MP can do very little, except raising the issue in the Parliament. And a question to the Parliament can easily answered and finished by just one sentence from the Secretary of State.
And firing a lawsuit against British government seeking to overrule a past decision is impossible. Matters involving diplomatic relations would be summoned to Foreign Office. Negotiation will go underground between China and Britain. The British government is willing to give up a certain legal disciplines for the sake of securing a result satisfactory to all parties. This is shown by the cases of the British government in the 1970s. Not to mention the case in 2016 is based on a very very weak legal base
I am sure i would be regarded as defeatist by you all. But dont be so hyper over a reply from JUST a MP... The majority of British politicians does not give a fuck to HK.