July's slide is over. Across the Costco filling stations we track in the UK, the average price of unleaded finished August at 154.2p a litre — up 3.5p, or 2.3%, from 150.7p on August 1, and diesel rose harder, up 6.0p (+3.5%) to 174.4p. Every one of the 21 stations that posted on both ends of the month got more expensive. The RAC had already flagged early August as the highest pump prices since the Iran war; Costco's club forecourts then kept grinding higher through month-end, unlike the supermarket mix that later flattened. It is the opposite of the July report, when Britain finally caught the global slide, and it sits next to a mixed month in the U.S., Canada, and Australia.
| Grade | August 1 | August 31 | Month change |
|---|---|---|---|
| Unleaded petrol | 150.7p | 154.2p | ▲ 3.5p (2.3%) |
| Super unleaded | 162.3p | 165.5p | ▲ 3.2p (1.9%) |
| Diesel | 168.5p | 174.4p | ▲ 6.0p (3.5%) |
National averages of posted prices at tracked UK Costco filling stations, p/L. The sample grew from 21 to 22 on August 20 when Sunbury entered. Snapshot of calendar August 2026 — this article is not updated.
A grind higher, not a spike
Petrol rose most of its 3.5p in the first week — 150.7p on August 1 to 153.4p by August 8 — then spent three weeks in a tight band before a last-week creep to 154.2p. Diesel never paused: 168.5p to 174.4p in a near-straight line, a 6.0p climb that left it at the month's high on August 29–31. The two lines move together because they are the same wholesale event; diesel simply has more of it to pass through.
Every comparable station rose
There is no fallers table this month. Southampton added 9p, from 146.9p to 155.9p, the biggest move on the network; Leicester added 6p, and Thurrock, Aberdeen, and Leeds 5p each. The cheapest unleaded on August 31 is a seven-way tie at 151.9p — Edinburgh, Haydock, Leeds, Aberdeen, Birmingham, Oldham, and Watford. The priciest cluster sits at 156.9p: Stevenage, Reading, and Chester — with Sunbury, which entered the sample on August 20, also at 156.9p. That 5.0p spread is tight by Costco UK standards; the whole country moved as a block.
| Station | Unleaded now | Month change |
|---|---|---|
| Southampton | 155.9p | ▲ 9.0p |
| Leicester | 153.9p | ▲ 6.0p |
| Thurrock | 155.9p | ▲ 5.0p |
| Aberdeen | 151.9p | ▲ 5.0p |
| Leeds | 151.9p | ▲ 5.0p |
Biggest August rises in posted unleaded at tracked UK Costco filling stations, August 1 – 31, 2026. All 21 comparable stations rose.
Cheapest unleaded right now
| Station | Price |
|---|---|
| Edinburgh | 151.9p |
| Haydock | 151.9p |
| Leeds | 151.9p |
| Aberdeen | 151.9p |
| Birmingham | 151.9p |
Priciest unleaded right now
| Station | Price |
|---|---|
| Stevenage | 156.9p |
| Reading | 156.9p |
| Chester | 156.9p |
| Thurrock | 155.9p |
| Southampton | 155.9p |
Posted unleaded prices at tracked UK Costco filling stations, August 31, 2026.
Why Britain climbed while North America chopped
UK forecourts lag wholesale by up to a fortnight, which is why July was still falling while the U.S. had already bounced. August is that lag arriving. Early in the month the RAC put petrol and diesel at their highest since the Iran war, with the Strait of Hormuz still restricting distillate supply Britain depends on more than it refines. Costco then kept repricing through the month — a 3.5p petrol / 6.0p diesel grind — because club sites pass wholesale through faster than the supermarket estate the RAC samples.
Duty plus VAT is still roughly half the pump price, which mutes the percentage move: official weekly road-fuel averages swing less in percentage terms than crude does. That is also why a 2.3% Costco petrol rise can sit next to a RAC headline that the national supermarket average later flattened: two different samples, same wholesale, different pass-through speed.
The fortnight lag is now working against you
In July the lag meant UK pumps were still falling after North America had turned. In August it meant Costco was still rising after some supermarket averages had stalled. If crude eases, Britain is owed a delay before it shows; if distillate stays tight, diesel's 6p climb is not finished.
Will fuel prices fall in September?
Only if wholesale does, and only after that lag. The EIA outlook still treats Hormuz shipping as the swing for global product cracks; UK diesel is more exposed to that than petrol, which is why it led both July's drop and August's rise. Costco's 154.2p unleaded and 174.4p diesel are month-end highs, not mid-cycle pauses. Haydock and Edinburgh at 151.9p remain the cheapest fills on the network; Stevenage, Reading, and Chester at 156.9p are where the month's grind landed hardest.
Costco pump prices are a snapshot of calendar August 2026 (August 1–31) and will differ at the pump today. This site is independent and not affiliated with Costco Wholesale Corporation.
