Instead of sealing the MDF, try a double layer of plywood, preferably Baltic Birch, Glue and clamp them together using cawls, flat lumber spans the plywood, clamps go on the ends to apply pressure evenly across the plywood. Cut an opening big enough for the base of the router in the second layer, or if you are going to use a metal mounting plate, cut the opening about half an inch inside the shape and size of the plate. You can then use screws through the seond layer to level the plate with the top. Cut the top opening the exact size of the plate, PLUS about the thickness of a playing card more or you'll never get the plate out.
I have attached a couple of images of a fence made from ply that are pretty cool. The first drawing is the offset, but slightly different than the simple double layer because you would cut a rebate or rabbet edge slightly deeper than the thickness of your plate.
The second is a Kreg leveling device you may be able to get over there. Or you can drill throught he second layer and fit a screw up through the bottom. File off the sharp end if you wish. Not expensive here, but easy to install, and you can cut the opening the full size, no lip is needed using it.
Glad you decided to jump in. The most important part of learing woodworking is to make stuff yourself. You'll learn from each project you do.
BTW, get yourself a dust mask when working with power tools. The router throws off a LOT of sawdust and it's not good for you. Take and post a picture of your table when it's done.
I have also posted a pdf of the 18 things I've learned over the past decade that accelerated my learning curve. It's about USA gear, but most of it applies to the UK, just different model numbers and brands.
The more I do, the less I accomplish.