Buy All Day Long, or P20  and get your wife to do it before you leave the apartment, then you wont need to bother anyone.

Seriously .....

it would be very awkward to ask someone.  I would try an all day and put it on before you go to the beach.  Remember though that if you swim you would need to top it up as they are more water resistant than waterproof.

Sean.

Seriously .....

Thanks for enabling me to start the day with a laugh. :)

But, just for fun, let’s give the benefit of the doubt and assume it’s a serious question.

I wouldn’t rely on Riemann P20, Ultrasun Sports or other any sun lotion other that’s claimed to provide ‘all day protection’ and I don’t recommend that you do either, especially if you’re heading south of the UK for your holiday, unless you have very dark skin (e.g. ethnic African skin).

If you’re 29 years old (as suggested by ‘close to hitting 30’ in your profile), I recommend that you regularly apply a good sun lotion yourself. If you’re able to clasp you fingers behind your back, as shown in the photograph on the webpage here: http://sportsmedicine.about.com/od/fitnessevalandassessment/qt/ShoulderFlex.htm, applying sun lotion or sun spray to your back is not difficult.

If you’re unable to clasp you fingers behind you back, I recommend that you see your GP, unless you have a known long-term physical disability.

If you have a known long-term physical disability or injury that renders you unable to clasp your fingers behind your back, I recommend that you politely explain this to a nearby adult naturist, as most naturists are compassionate people who would be willing to assist in this situation.

Answers to your specific questions:

...is there any single people who have gon e to a beach ...

Yes (e.g. me). More than one beach, in fact.

... have you asked any one to put sun tan lotion on your back...

Yes, but only people I already know.

... id rather ask than to go with out...

‘Asking’ and ‘going without’ are not the only possible options.

... what you do...

I either put sun lotion on my own back or accept a spontaneous offer from someone else to do so if I already know that person.

...has this happened t you...

Yes. I’ve been on holiday, some people call me a naturist and my skin type means that I need to use sun lotion.

...what type of people would you ask, families group of friends...

I wouldn’t ask anyone I don’t already know and people I do know usually spontaneously offer before I need to ask.

...as im a guy maybe go to the guy only section of the naturist beach. 

Not applicable, but even if there was such a thing as a ‘girls only section’ I doubt I that I would proactively ask anyone there to apply sun lotion onto me.

I do as Katya suggests and do it myself if necessary. Not everyone is going to quite that flexible but if you use a spray protection it might be easier to get it where it needs to go.

Always a risk asking a stranger in case they take it as an invitation for physical contact and then it could get embarrassing.

I use a thing like a small paint roller.

Buy All Day Long, or P20  and get your wife to do it before you leave the apartment, then you wont need to bother anyone.

As I see it this is the only useful advice.

I personally have used P20 various spf ratings over the years and after applying first thing with other half doing my back it was one less thing to worry about.

You can purchase a lotion applicator with long handle for oils and cream.

I use the same as Mickthefish ---small paint roller "perfect"
A.

Buy All Day Long, or P20  and get your wife to do it before you leave the apartment, then you wont need to bother anyone.

I thoroughly agree with StevieLorna.
I have asked my wife to apply P20 on my back for as long as P20 has been on the market.  I have spent many hours out in the sun, going for a swim after one application and never had a problem.
One caveat, if you want an even all over tan that you apply this stuff evenly and be thorough to make sure there are no gaps.