Have you thought about Chinese studies/Globalisation/Middle East?
I think these three options are pretty legit.
If you are not British, you cannot apply for to M15 related work, not sure how terrorism is going to be a helpful focus for you, whereas trade wars and deglobalisation are more relevant these days.
In future, if you cannot enter politics, you can still try to be a civil servant in department of trade or be in the supply chain field.
UK politics is far more complicated than you can imagine, I know a guy who worked with Nicola Sturgeon, he started out on a grassroots level, joint SU/labour student club/trade union/NUS, championed for rent-free zone/anti-arm trade sort of things, he is soon starting a position at Brussels. Pretty standard path for most folks I know to be fair.
You have to really get involved as early as possible, volunteer/work for the MPs as well. As student representatives, it is often that you can be invited to speak on regional/council/parliamentary committees, alternatively, you can apply to be a trustee/observer of charities, just to build network locally or to have something on your CV. They love "diversity", you do have to make an effort to mix with the right "crowd".
Some other people I have seen applied to direct internships at the House of Common. With notable achievements, I personally know two people who got invited to a "masterclass" at Downing street this year as young leaders. I used to work in politics but I absolutely despise career politicians, it is not a life for everyone.