I have a set of double wood doors that need to be secured permanently

all other doors in the area have been ungraded to steel multi lock doors, and the security on the wood doors needs to match.

I could fit another double steel door and just lock it but im wondering if anyone can suggest a cheaper option ?

Is a secure steel blanking plate a real thing ?

When you say permanently secured, do you mean seal them up or do you want to be able to use them as a door still?

You could put a thin gauge steel plate or plywood on the inside of the door but that will add to weight and maybe to much for the hinges. If you are not going to using the doors then insert screws through the door into the frame, floor and where the doors join.

If you mean properly seal it up then concrete blocks would be very substantial and not that expensive. Maybe £150 for materials plus half a day or less.

When you say permanently secured, do you mean seal them up or do you want to be able to use them as a door still?

Yes seal them up, never to be used again

It can be done.

Thin galvanised sheet over the external face, bonded with adhesive on to wooden door. Thin angle around edge so you can’t get a lever under, fix the angle into the edge of the door with long screws and bonded under with something like Sikaflex EBT or CT-1. If fixing on the face fill the heads with JB Weld or use security screws.

Upgrade the hinges if needed, fit hinge bolts. Fit flat steel strip on inside of both door and frame on all sides where there are either hinges or locks fitted, both bonded and screwed into door. Like a London Door Bar, google it. The strip bolsters the wood and should prevent the wood from splitting.

EDIT – In that case just block it up, as above.

I’d just brick it up using steel plates into the existing walls to get a good bond.

…and park something in front until the mortar’s hardened

Brick it up…if you want it to look good from the outside, and the doors are in good condition, leave them in situ and just build a single skin inside the doors. Doors don’t need to be locked at all, thief opens door with handle rather than crowbar, faced with brick wall, walks away (to your main door, presumably thats already secured)