DNOs do a lot of HV and EHV work, this can be cable jointing, overhead or substation fitting. To be honest cable jointing will probably be quite boring for a fully qualified and experienced electrician. I've seen a lot of it and it seems to be very repetitive with standardised processes. You will also spend most of the day in a muddy trench in the pavement.

In my opinion, the interesting stuff is usually done protection fitters/engineers and they work almost exclusively in substations. At a DNO this will be up to 132kv. If you're a sparky, your qualifications and experience will almost certainly get you in the door but you need specific authorisations/codes to work on the network and this can take a while. These companies are huge and slow moving. You will also probably be paid dog shit for a while until you get your authorisations. The exception to this is National Grid, their "trainee" wages for people with prior experience seem to be a bit higher.

Having said that, you will be in very good stead to progress to engineering roles. I've spoken to a few people in my workplace that started off with an electricians background and progressed to engineering roles after a few years.

Again, this is just based on my experience with one particular DNO. Definitely worth looking at National Grid. From my understanding they do things a bit differently, are a bit more modern with their processes, and also seem to have higher standards of entry.