Best wood for exterior use

For something like that you want a material that can handle moisture and movement, not just standard timber.
Best affordable option would be exterior grade plywood (WBP/marine ply). It’s stable, won’t warp as easily, and takes paint well. I’d go minimum 12mm, ideally 18mm, then seal all edges properly before fitting and finish with a good exterior paint.
Another good option is treated softwood, but it’s more prone to movement and splitting over time unless it’s well sealed and maintained.
If you want a longer-lasting, low maintenance solution, you could also look at uPVC panel inserts, which are designed for doors and won’t rot at all — often not much more expensive once finished.
Main thing is whatever you use, make sure:
All edges are sealed properly
It’s fixed securely into the door frame
Finished with a proper exterior coating.

Hardwood Plywood Board – very affordable, strong and easy to cut to fit the opening. Good for interior use and can be painted or veneered.

Good Affordable Choices

Wickes General Purpose Spruce Timberboard – A larger board that’s easy to cut and fit as a flat panel replacement.

Solid Pine Furniture Board 18mm – Solid pine is easy to work with and finishes well if you’re planning to paint or stain.

Knotty Pine Furniture Board – Similar but with a more rustic look from the knots; a good budget choice.

Wickes PSE Redwood Timber – A straightforward softwood board that’s cheap and workable.

Treated / Weather-Resistant Options

Treated Rough Sawn Whitewood Spruce Timber – Treated wood helps resist rot if the door gets wet often.

Pine Timber External Door Frame Sill 145mm – Useful if you need a shaped piece or sill detail.

Cheshire Mouldings Smooth Square Edge Pine Stripwood – Good for clean trim around a panel.

A Slightly Higher Grade but Still Reasonable

Solid Sapele Hardwood Timber External Door Frame Sill – Hardwood like sapele is stronger and more stable than softwoods if you want a longer-lasting panel (still more affordable than exotic species).

Painted marine or hardwood ply.

Hardwood is the best option, always treat or primer and paint to protect the wood
But a exterior plywood, or t& g are other options

Thanks for the detail.
To cover the bottom section of the back door (after removing the glass), you’ll want a timber that is suitable for exterior use, resistant to moisture/rot, and reasonably priced.

Here’s what I’d recommend:

Choose a piece of softwood or affordable hardwood that is treated (or you treat it) for outdoor use. For example: the softwoods pine or fir are budget-friendly, though they need good sealing and painting.
Hotian
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Hotian
+2

Make sure you seal all faces, especially the bottom and end grain, to prevent moisture ingress.

Cut the piece to fit snugly, fix it in place (with appropriate screws or adhesive) so it forms a solid cover rather than a weak patch.

Finish with an exterior-grade paint or stain + varnish to further protect it.

If your budget allows, you could upgrade to a more rot resistant timber (like white oak or cedar) but that will cost more.
yourhomesecuritywatch
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If you like, I can pull up 3–5 specific timber products available in the UK (with prices) which meet this spec so you can compare cost/quality. Would you like me to do that?

Since the bottom section of the door is exposed to weather (rain, splashback, possibly damp from ground), moisture resistance is key. Woods with tighter grain or natural rot/insect resistance are better.
aprodoor.com
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Budget matters (“best but affordable”), so choosing a good value softwood and applying good protection is a pragmatic approach. For example, pine is noted as low cost though less durable unless well-treated.
Hotian

The job is a retrofit (covering the bottom where glass removed) rather than full door replacement, so you don’t need the highest spec premium hardwood unless you want the look and long lifespan.

Since you’re looking for “best but affordable”, I’d suggest going with a treated softwood option as a sweet-spot (for example the treated softwood or whitewood stick above). Then:

Seal all faces and especially bottom/ends.

Use a proper exterior finish (paint, stain + varnish) to protect.

Install with good fixings and ensure any joints are well sealed against water ingress (especially since you removed glass and may have exposed frame or edges).

If you ever see rot or moisture creeping back, consider upgrading to hardwood next time (but for now you can keep cost reasonable).

Plywood

Marine Plywood

WBP Exterior play wood

Either a good treated timber or exterior hardwood

Treated timber

Sapele

3/4 inch plywood or MDF (Medium Density Fiberboard)

Plywood

Hi, best option would be marine plywood, but cheaper option would be moisture resistant MDF. Obviously both will need painting with external paints to make sure no panels left exposed to weather.

I would suggest treated Wood .

Plywood

Marine grade ply wood

The best cheap wood to cover the bottom of a back door where glass was removed is exterior plywood like WBP or marine ply. It’s strong, weather-resistant, and easy to paint. Cut it to size, screw or glue it in place, seal the edges, and paint it with exterior paint to protect it from rain and damage.