Ceramic tiles over wooden floor

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My original wooden floorboards in the whole of the downstairs floor (large victorian semi) need replacing because they're in bad condition and it's really drafty. I was thinking about insulating under the floor in the dining room and sitting room which would involve getting rid of the original floorboards altogether and replacing it with engineered oak. Rather than doing the same in the kitchen and large entrance hall I was thinking about tiling those spaces. Would the tiles go on top of the existing floorboards? What if we go for underfloor heating? What would be more cost effective, under floor heating or under floor insulation? The prospect of removing all floorboards in the whole of the downstairs, insulating and putting engineered oak is so daunting and expensive I just don't know what option to go for. So I thought if we did the tiles are first perhaps it wouldn't be too disruptive. But now I'm rambling...

Usually you don't replace the boards with engineered oak, it goes over the top of the existing floorboards.
Tiles would also go over the top but you would need to also put board underneath (usually ply) to reduce the flex and avoid the tiles cracking.
You also need to be careful about the size of tile, grout and bond pattern to avoid cracking.

We tiled our bathroom, we had to fit ply over the floor boards to give a good solid flat floor then fitted electric UFH then tiles. If you are looking at doing downstairs and maybe fitting UFH, you would need to insulate under the floor also so you don't lose your heat. you may also find that the floor after boarding and tiling would be higher than that of adjoining rooms - not an easy job but a lot warmer ! Good luck

To tile on a wooden floor, you usually screw down first a layer of 18mm ply to make it more rigid.

If your boards are in poor condition I would take them up and lay 25mm ply, wuth noggins under all joints, and lots of screws. Retighten the screws after a couple of weeks. Clear up all underfloor rubbish, sweep out the airbricks, insulate pipes, and insulate between the joists while the floor is up.

You will in future be unable to access the plumbing and wiring under the floors you mention, without ripping up the applied coverings. You will eventually regret this.

Thank you everyone for your replies.
Piglet, are you saying I will regret tiling? The floor needs replacing regardless. I was just hoping to avoid some of the underfloor insulation in the kitchen and hall but from what you're saying it's unavoidable?
What pipes would I need to access? I mean, if you remove the existing floorboards and replace it with engineered oak, pipes and stuff will be covered anyway, right?

You will have got some electricity cables and probably some radiator pipes under your floor.

For best results, have any additional sockets, doorbells, internet cables, main bonding, doorbell wires, new waterpipe, speaker connections etc done while the floor is up, insulate the pipes and move or fit any radiators.

electricians and plumbers are always sad when householders tell them to do repairs by magic without disturbing the flooring.

If you have a good crawl space, you might be able to put in an access hatch.