Langley -
I rather think the cost of the Weber is entirely dependent on where you buy them.
These are using Amazon Prime for the "free" delivery, for a total of $208 or thereabouts.
According to this site, GBP 100 in 1988 is now worth 234.
GBP 234 is more in pounds than the cost of the three new grills in dollars! Ergo, you either bought better models than the comparison above, didn't buy them in the PX/BX/USA proper, otherwise overpaid, or they've simply become cheaper in real terms.
Even if one bought the Platinum models of the three grills today, I doubt the total cost approaches the $400 or so that your 100 pounds in 1988 is worth today. Assuming of course you buy them from a US source.