If you can't get there in person here's the next best thing!
The website is distinct from the National Gallery's, and is also a great resource on lesser known artists. It houses over 10,000 portraits and is a real treat.
The link is
www.npg.org.uk
If this doesn't get you through, as it can be temperemental, try
www.npg.org.uk/live/index.asp.
Enjoy!