Please ID this beetle

My wife spotted a large (about an inch and a half) beetle in the garden yesterday morning and managed to snap a couple of pictures, can anyone help identify the species please?

Picture1
Picture2
Picture for scale

We live in the UK in the west country where it is currently early spring time. Lots of bugs (ants, bees and flies) are starting to wake up after winter plus it has been raining a lot lately so I guess it woke up and is searching for food. It’s quite rare for us to see this kind of thing in our garden.

I took a look at loads of beetle images on the web last night but couldn’t really identify it, the female stag beetle is pretty close as are various snap beetles.

She said it was very slow moving and only saw one but she is now convinced we are about to be taken over by radioactive cockroaches :eek: Please help put her mind at rest!

Dytiscus marginalis, female.