I want up to 5 polite sample letters to turn down an invite to a
wedding I don't particularly want to go to. Want to stay friends. Country is UK.

Boxmoor,

There appear to be only three polite ways to refuse a wedding
invitation. The first is simply to check the appropriate box on the
reply card.

If there is no reply card, you can reply formally or informally. The
following link includes an example of each.
Bed Bath and Beyond
Regretfully Yours: How to Gracefully Turn Down a Wedding Invitation.
http://www.bedbathandbeyond.com/regShowWWTip.asp?order_num=-1&tip=wedding_regrets&tiptype=Guests

The following link includes another sample of an informal decline
(very similar in form to the first link), but also includes some do's
and don'ts.
The Wedding Channel
Sending Your Regrets
http://www.weddingchannel.com/ui/buildArticle.action?assetPath=/templates/Articles/NonContentSet/Guests/article_1794.vm

Have a pleasant stay with your friends!

- Hammer

Search Strategy
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"wedding invitation"  decline

I don't feel you need five letters, only one polite one that conveys
the message that you can't attend. Simply saying something like:
Thank you so much for your invitation. I'm regretful to say that I
will not be attending due to personal reasons. I wish you a very long
and happy lifetime together!

Let them wonder what the personal reasons are. You are under no
obligation to attend simply because you were invited, and I highly
doubt they will ask "why" you didn't attend considering you already
stated it is because of personal reasons.

Dear (Insert first names here), 
I was absolutely delighted to receive an invitation to your upcoming
special day. It means so much to have been spared a thought,
especially when I know how busy you both will be in the next few
months, ensuring that everything goes so smoothly.
However, I'm afraid that I must decline, as circumstances within my
own personal life have prevented me from attending. I will, however,
also spare you a thought on this day, and would like to wish you both
my deepest regrets, but also my best wishes.

Kindest Regards,
(Insert your name here)




-=-=-=-=-

That should be personal enough to say thank you, and show that you
appreciate the sentiment anyway!