I would not say any one agency is worse than another. To be fair to them all, I honestly believe they try to pass on mail if possible, and any non replies is probably down to accidents with misdirection/mail or genuine non replies from the celebrity after they have had the mail passed to them.
I just checked my spreadsheet and for the two agencies you mention -
United Artists - 14 successes, 11 non replies
Curtis Brown - 32 successes, 24 non replies
Pretty similar reply rates.
That also includes some recent requests, and some requests where I have tried more than once which backs up the suggestion that it is the celebs more than the agencies that result in non replies.