TV Comic: 1377 (5th May 1978) & 1378 (12th May 1978) — Skateboard Iron-On Action Transfers
"Iron-On Action Transfer", eh? Hmmm. Well, Letraset did print iron-ons occasionally; & the term "Action Transfers" was a trademark. So I'm inclined to think Letraset probably did print these — although it could all just have been an innocent mistake in the wording.
Letraset isn't mentioned in the interior text, but then you wouldn't necessarily expect them to have been, even if they were responsible.

Photo courtesy of tis_santa

Photo courtesy of fambridge-comics
Photo courtesy of phil-comics
Since the only text on the transfers is "© 1978 Polystyle Publications Ltd.", there's no confirmation that they were printed by Letraset.
While we're at it, in issue No.1427 (20th April 1979) you could get a free "Incredible Bulk" sticker. Probably not by Letraset… but I'm still mentioning it, just in case. They certainly had the technology to do so!

Other pages in this article about Free Gift Transfers in Comics:
- Cor!! No.2, 13th June 1970 — Anglo Bell Boy Double Treat — Somportex Combat Bubble Gum →
- Michelino: 1, 2 & 3, 1971-1972 (PR130) →
- Look-In No.8 (of volume 7) week ending 19th February 1977 — Action Transfers (SAT Carnival) →
- Wall's / Look In No.11 (of volume 8), week ending 11th March 1978 — "Star Wars" Transfers Offer →
- TV Comic: 1377 5th May 1978 & 1378 12th May 1978 — Skateboard Iron-On Action Transfers — this page; see above ↑
- Look-In No.40 (of volume 8) week ending 30th September 1978 — Free Kalkitos Fun Transfer Kit →
- TV Comic Nos.1452 & 1453, 12th & 19th October 1979 — Hanna-Barbera Transfers →
- Whizzer and Chips, Monday 13th October 1979 — Free Gift Kalkitos →
- Marvel UK: Doctor Who Weekly Nos.1-4, October 17th - November 7th 1979 — Transfers by Dave Gibbons →
- Marvel UK: The Empire Strikes Back Weekly No.118, May 29th 1980 / Dairylea — Star Wars Transfers →
- TV Comic: 1530 17th April to 1533 15th May 1981 (4 issues) — Popeye Transfers (LP147) →
- Eagle: 17th December, 23rd December 1983 — Ultraman Poster & Transfer (LP221) →
- Marvel UK: Transformers Nos.1-3, 20th September - 31st October 1984 — Free Action Transfers →
- Marvel UK: Spider-Man No.607, 27th October 1984 — Free Rub Down Spider-Man Transfers →
- Splat Comic No.1, 27th February 1985 — Terrifically Terrible Transfers (Tattooze) →
- Marvel UK: Secret Wars No.1, 27th April 1985 — Free Secret Wars Rub Down Transfers →
- Marvel UK: Return of the Jedi Nos.97-99, 27th April - 11th May 1985 — Free Ewok Transfers →
- Marvel UK: The Spider-Man Comic Nos.634-635, 4th-11th May 1985 — Free Action Rub-Down Transfers →
- Katy No.1, 31st October 1986 — Colour Transfers Free Inside →
- Disney Magazine (UK) & Micky Maus (Germany) — 1984-1991 →