The fact that there's a mobile pass does not mean that all participating companies suddenly have the possibility to book online reservations. And the type of pass (paper or mobile) doesn’t make a difference for the reservations.
For Great Britain, you can make a reservation via GWR using these steps. Reservations for domestic day trains are optional. You can also do that at any ticket office in GB. Seat reservations in GB are free of charge.
For Spain, online reservation is not possible. You can book:
- at a ticket office in Germany of via phone (+49 30 2970) but the reservations will still be on paper, so you'll have to collect them in Germany or have them sent to you via the post.
- at a ticket office in Spain
- by phone via RENFE, with limitations, see this page (scroll down to Spain)
Pass holder seats in Spain are not quota-controlled, so booking a few days in advance is usually enough.
For Portugal, you can only book at a ticket office in Portugal. Here also no limited quota.
For any high speed trains in/to/from France though, quota for pass holders are limited.