Not specifically. Lot of self-defense classes teach similar techniques. Just a basic grab to a sensitive area, but not one that will always work.

If you can reach, yes, that's not a bad move to make them let go. However, if they have a strong grip around your upper arms and chest, you probably won't have the reach to get down and under. The most commonly taught self-defence move from this position is to stamp on their feet with your heel, and to scrape down or kick their shins. Hurts like hell.

That's probably not the best advice because if someone is grabbed from behind, it would be difficult to turn around to attack the groin. It would be even more difficult to attack the groin with ones hands grabbed from behind. The most effective groin attack technique for that position is kicking the attacker's groin by lifting the heel up to the groin area as hard and fast as possible.

There are other self defense moves for that position that don't require groin attacks.

I would suggest trying to enroll in a self defense class if you can. I have taken a class myself and it was great. They teach you about not only self defense but how you can defend yourself in any situation. They also decipher a lot of the movies that view women as the victim- and that is not the end of the line if your captured or fall down on the ground.
You can use your voice as well by saying 'NO!' very loud with all the strength you can muster. By doing this your assailant will be caught off guard and anyone near by might have over heard. Your assailant views the victim as weak and easily manipulated. He/she thinks they can easily over power you and doesn't expect much of a fight. They don't really expect a fight or want a struggle either.
So you have to defend yourself with everything you have. You should get a bigger purse or something. Maybe you could carry pepper spray or even an electric tazer. But keep in mind anything you use against your assailant can be used against yourself – so make sure whatever you use you know how to use.
If your ever grabbed by the arm you can also maneuver out of that grab, or punch him with the side of your elbow in the face. The face is very fragile and the neck is as well. Punching in the throat or literally poking very hard into the eyes will hurt your assailiant enough to get away.
Before running away make sure to check that hes down and hasn't pulled out any weapons and then run.
I would recommend doing a self defense class. They teach you several things that will help you in any situation.

What about a pepper spray or use you're keys. I heard that placing your keys in between your fingers so they look like spikes poking out of your hand is a very nasty way to stop the bastard! Keys can do some serious damage! or use you're fingers and gauge his eyes! or destroy his testicles!. I took self defence years ago Tie kuwondo but I'd still be scared if I'd got attacked. You need to be confident in defending yourself and a lot of people are unable. I would suggest you ask someone like a friend too pick you up from work instead or get a cab. It's not worth risking your life over a job.

panic alarm -you can keep in pocket – is not a weapon but makes a big noise – keep one in pocket , on in bag

can of mace…legally purchased firearm with applicable permitting and training…electronic stun gun…quick extending metallic baton

You can buy personal attack alarms off the net, and they are legal in the UK. Pepper sprays and tazers are not.

The attack alarms emit a VERY loud tone, which would put off anyone and wake up half the street. They cant be turned off easy either.

Thats the best legal way. or you could go to kung fu class or somthing.