I was catching up on my reading last night by reading the December issue of Stamp Magazine.
I particularly enjoy reading the Rear-View Mirror feature each month. In December the article was talking about British thematic issues depicting flora and fauna and mentioned the 1973 trees issue which I have discussed below:

The first issue 1973 British Trees stamp series was issued on February 28, 1973.It was a notable collection in the history of British philately. This series was part of a broader initiative by the Post Office to highlight environmental and conservation issues, aligning with the “Tree Planting Year” campaign.

The first issue in this series featured the Oak tree, a symbol of strength and endurance in British culture. Designed by David Gentleman, a renowned illustrator and designer of many British stamps since 1962, the stamp was printed by Harrison & Sons Ltd using the photogravure process. The Oak tree stamp had a face value of 9p and was available in presentation packs and first-day covers.

The Post Office Philatelic Bulletin provided detailed information about this stamp issue, including its design process and the significance of the Oak tree in British heritage. Collect GB Stamps, an online resource for British postage stamps, also offers information on this series, including images and descriptions of the stamps.

Curiously the series ended in 1974 with the 2nd issue depicting the Horse Chestnut Tree.

For more details, you can explore the Collect GB Stamps website and the Postal Museum.