The transfer time in London is no problem at all; you just need to cross the street between the two stations. 

The time in Paris may be a problem with option 2 if you are completely unfamiliar with the situation in Paris (in fact it’s just a 10 minute walk but there may be a waiting to get out of the platform if you are at the wrong end of the train and you may need some time to orientate). But normally even this should work, too.

The mandatory reservation to Germany is 18 € via Paris and just 3 € via Bruxelles (mandatory reservation just from 1.6.). Normally, the control takes place in London. You may even do a seat reservation for the next ICE from Bruxelles at 16.23 (just in case) and it’s still cheaper than via Paris and you’d still have an earlier arrival. Personally, I’d go for this.

@Hektor Thanks for the reassurance! I was afraid that the checks in London may take too long till I miss the Eurostar train / the LNER train from Edinburgh gets delayed for some reason.

I am also planning the contingencies for strikes. In the event of strikes like the one recently, do I have to rebook a seat on Eurostar? Since I would definitely miss the train out of London due to LNER cancellations

Yes, it is definitely cheaper to go via Brussels. Are delays often for this train? I am not in a rush, but would just want to get to my accommodation by the end of the day since I am activating a travel day for this trip.

You just need to be there at check in time. According to Eurostar, you should be there 60 minutes before, the gates do close 30 minutes before.

Of course you need to rebook. But I wouldn’t worry too much about strikes. They won’t be announced the day before. 

As written before: Invest 3 € for a second reservation for peace of mind if you wish to. Normally the connection should work but it’s a tight one. But as also written before: Even the next ICE is earlier in Frankfurt than your other options. 

That London connection doesn’t give you any time for delays or cancellations on the East Coast line (certainly a lot less time than I give that London connection). I’m not sure whether EuroStar let you move to a later train if you miss the connection but perhaps someone else knows? 

They do.

Thanks! So if I miss the connection at London due to delays/long queues at the station, I can get a seat at the next train without additional reservation cost? (provided that there is a seat)

Yes! That’s what I am afraid of actually, do you think 1h is enough to pass through security and everything at the station?

From frequent Eurostar travellers, I've seen multiple reports that the best time to join the queue is 1 hour before departure time. If you join earlier, then you just make the queue longer and the waiting space more crowded since the previous train is still there and might not even have started boarding. If you join the queue later, then keep your eyes and ears open for signs and announcements about your train. Eurostar employees often try to find passengers in the queue for a train for which the check-in almost closes, to fast track them through security.

@rvdborgt Okay, thank you! I guess I would probably go with the first option and transfer via Brussels.

Because I was concerned about missing the connection between the North East Coast LNER train to Kings Cross and the EuroStar from St Pancras I contacted EuroStar for advice on what they would do for me should I be delayed arriving in London.  Their response is above.