Hello Akilan!!!
Although I'm not an expert in British standards or EN 287 & 288, there is a very good website at: http://www.gowelding.com that might bring you closer to jusitfying that the AWS is at least equivalent (most likely as stringent or more in required testing) to what are now being replaced by the new ISO standards if I remember correctly...

There is a wealth of information in this website pertaining to your query, and if you ca'nt find the answers that you're seeking then, you can e-mail the person (I forgot his name) that runs this site, who has considerable experiences with British standards, and by the way, he's a registered European Welding Engineer...

Another place where you might find the justifications you seek is:
http://www.twi.co.uk (The Welding Institute of the U.K.).
Have you contacted the AWS for help???

I hope you find what you're looking for, and I'm sure others that frequent this forum will help also, unless that is, if you're taking jobs from us "Yanks" here in the good old U.S.A !!!
No disrespect to all of you gents from the south or Boston Red Sox fans but, that's what the brits call us...
GO YANKEES!!! May the BEST TEAM WIN!!!

Respectfully,

SSBN727 Run Silent... Run Deep!!!

That's one of those things that we here in Brazil call "frescurice" (literally, freshness). A "fresco" (fresh) guy, in the sense Brazilians mean, is someone who has unreasonable requirements, such as your client, who doesn't accept AWS standards unless you demonstrate him that they're equal or better than British ones.
I'm not an American nor I live in the U.S.A. I've been only once in the States, as far back as 1978, taking part in a technology transfer program which lasted 6 months, in the offices of Badger America Inc., an engineering firm located in Tampa, FL, which, as far as I know, doesn't exist any more.
But, from the beginning of my engineering career, back in 1961, I've worked almost exclusively with American standards: AWS, ASME, API, ASTM, ANSI, CEMA, AWWA, ISA, AGMA, AISI, AISC .............., so I happen to be quite familiar with them.
If I were you, I'd tell your client that all over the world there are dozens of nuclear and hundreds of thermal power plants, oil refineries, chemical, petrochemical and pulp and paper plants, thousands of miles of oil and gas pipelines, hundreds of buildings and ships etc. etc. which were welded to American standards and didn't bust nor sank nor fell apart.
Isn't that a clear demonstration that American standards are at least as good as European ones? If this argument doesn't convince your client, tell him that Mr. Giovanni S. Crisi, from Sao Paulo, Brazil, says that his requirement is a "frescurice" and that he's a "fresco".
Giovanni S. Crisi
Sao Paulo - Brazil