I have to say according to that legislation it's not quite as simple as simply being illegal to open someone else's mail - the practice's legitimate excuse could simply be they receive and open dozens of letters a day, 90% of which are correctly addressed and as soon as they note that it is someone else's mail they deliver it to the correct address.
So it's neither intentional delay or opening post without a reasonable excuse, nor are they intentionally acting to anyone's detriment
Best bet is to try and speak to your postie who keeps messing it up, or just keep on at the surgery to re-deliver - maybe suggest that they keep a box under the desk in reception they can just put the mail into and you'll pick it up, something along those lines.
You'll also note that it is arguably perfectly legitimate to open someone else's mail if it is delivered incorrectly to you - you aren't delaying the transmission as it's been delivered to you anyway, and further that if you open it to, for example, try and find a telephone number if the address on it is wrong, then you aren't acting in any way to the person's detriment so you aren't breaking the law for that reason either.