The UK has been included in Eurail/Interrail since 2019.

Reservations are not compulsory on the vast majority of UK trains.  Exceptions include ‘Caldeonian Sleeper’ and ‘Night Riviera’ that are combined sleeping and sitting car overnight trains, rspectively from London to Scotland and Penzance in England’s West Country.

Eurail/Interrail passes are similarly valid for use on Northern Ireland Railways:

https://www.interrail.eu/en/plan-your-trip/tips-and-tricks/trains-europe/railway-companies

INTERrail-applies to NL-has always been valid in UK-jus EUrsail-for outer continentals, not-this was unified 3 yrs ago. There was even a special clause when UK started to privatise/contract out its former Britsh Rail that all would accept the passes. The major prob in UK is not RES-is not needed, and if wished for free at counter,m but the big amount of strikes-every few days here&there

There are not ‘strikes every few days’ in the UK.

There were three strike days recently in quick succession, though not consecutive.  On those days, most if not all rail operators ran trains: for instance, a friend travelled on LNER from London to Durham and return, though frequency was markedly reduced.

There is one strike planned by a separate union on 25 July 2022, and media tells me one operator (Greater Anglia) may have had another, separate recent strike day.

Amy strike is regrettable and a huge inconvenience for travellers, but let’s not exaggerate.

If you are citizen in the Netherlands, Belgium, France, or Great-Brittain) and you want to travel from/to UK with the Eurostar and you trip starts in your country of residence, use absolutely one of you two inbound-Outboundtrips, or you need to pay fot a ticket for the whole trip, what expecially in Standard First might be very expensive. In myu opnion, this may be useless, because a big part of the trip might already paid by Interrail (and so the same with travelling with Thalys Paris-Amsterdam/Germany), so with buying a ticket for Eurostar, the part outside of your country of residence is paid double. Simple solution: let Eurostar sell online-tickets with partial-validity, special for the group who need one.

EUrail-group might forget that most of the people with a 2-3 Month-Pass may return to their home-country for 2 of more times, (think about work, marriage, funeral, washing-clothes, solving some cases at home, etc.). Most of the people with a 1-2-3 Month-pass don't travel continiously through Europa for the whole time, so it's unbelievable that EURailGroup have done noting for this group.

I have EURail-Group advised to do something with this, (like offering the option to buy more inbound/outbound-passes for an X-amount, or let choose the traveller to use an inbound-travel, or not if he starts or ends his trip in his country of residence and in posession of a valid-ticket of abonnement for the part in his country of residence, (it's up to the train-staff to verify this, not EU-Rail-Group, isn't it? So EURail-Group, this option into the app, instead taking-place on the seats of the train-staff, please!).