I worked for a few years in a few EU28 countries. According to the principle of aggregation of periods each country will contribute to my pension.
I am only 27 and it doesn't seem likely that I am going to retire any time soon and the shape of the European Union once I retire might be very different from today. Brexit will probably happen eventually.
In the year 2060 or so, will the UK still be obligated to pay my pension, even though they are no longer part of EU?
My best guess is "probably not".
My guess is that if you retire in (say) Germany, you will have to claim a UK state pension directly from the UK (separately from the pension you claim from the German government which arises from all your time in EEA countries).
To qualify for the New State Pension in the UK, you need to have 10 qualifying years on your National Insurance record. At the moment, time spent in the EEA effectively counts - I expect that for people retiring after Brexit (or possibly after the transition period), it won't. Thus if you haven't worked for 10 years in the UK, you won't get anything from them.
Either way, I strongly urge you to save into private pension schemes in addition to the state pension - state pensions are likely to get worse, not better.
