Where do I start?

Hi Anna,

You can put underlay down.
It does help with uneven track and reduces the noise a little bit, On my OO layout I covered the whole baseboard in cork tiles, much cheaper than just using strips of underlay below the track.
On my N-gauge layout I fixed the track directly to the baseboard without using any underlay!
I have used the foam underlay, but took it all up, it just doesn't look very good, proper ballasting is really the only way to go, IMHO.

As for not using the TrakMat...why it's a great way to start building a layout IMHO

The only addition I would make to it, is to install another crossover at the back of the layout behind the signal box.
Once a loco has driven forwards from the inner track to outer track on the front cross over the only way to get back is by reversing from the outer track back to the inner
You can fit another cross over behind the signal box position without effecting anything, you would just take out a couple of short straights, so that when on the outer track going forwards (anti-clockwise) the cross-over would take you from the outer track back to the inner track.
Thats about it really, in the Hornby plans book there are another 3 extensions that can be added to this layout using the two sidings that go into the engine shed, down the right hand side.
The shed is removed and another board put at the back of the layout, so it can easily be expanded upon.

If you click on the "my layout" link below in the signature bar, this is a complete TrakMat layout including all the equivalent extension packs only it's in N-gauge and not OO.
It should make it easier to see what I mean about the cross-over.

Any questions just ask away.

Baseboards!

I've used 12mm thick MDF, although this isn't popular with some people, it is prone to warping, although I've had no problems.

Most folk recommend plywood same sort of thickness, build a frame from 2inch x 1inch timber glued and screwed with cross braces then glue/screw the baseboard to the frame.

This photo shows the frame where the cross is, the baseboard is on top of this, the sides have been built quite substantially so it can incorporate the folding legs.

You can just use the 2" X 1" timber all the way around the edge of the board with cross bracing every 300mm or so, best to mark out the track on the baseboard first so the bracing can be fitted either side of where you may want to fit point motors.
These fit onto the points so they can be operated electrically.
This shows the hole cut out of the baseboard (note the cork covering the baseboard!) for the point motor to fit through, the point motor is attached directly to the point itself, you can fit the point mtotors on top of the baseboard then cover them with a hut, choice is yours I just I'd let you know the choices.
You need to think about this as it's easier to cut holes in the baseboard now, than if you get way down the build then decide to fit motors.

Hope it helps