Tricky tricky situation...

But how do I declare the work permit null and void before I start the four week count down? And surely I can return after to the uk on the tourist 3  month entry period if I have to? How deos one 'end' a work permit?

You don't "declare" it...it just becomes void 4 weeks after your job ends.  I expect your former company will do some sort of paperwork to let immigration know they are not sponsoring you any longer.

You need to be careful with this.  Immigration doesn't give you strikes....one screw up and they tend to call you out permanently.  If they see that your visa has expired, but you didn't leave when you should have (and you will have to submit your passport to apply for any visa....they will see from the date stamps if you have overstayed), they are less likely to trust that your situation is legitimate.

Don't assume you can just return for 3 months, especially if you've overstayed a visa.  They aren't obliged to let you stay even one day if they think you aren't here visiting as a tourist....and if you've been here for extended periods this might get the immigration officer's suspicions up.  Tourist visa waivers aren't a right....they don't have to let you stay even if you feel that technically you're within the rules.  It unfortunately doesn't work that way.

You need to either a) get back to the US as soon as possible before they catch up with you and you ruin any chance of getting back to the UK and your beloved anytime soon, or b) contact a soliciter that specialises in visas and immigration.  Your situation sounds pretty complicated!

But how do I declare the work permit null and void before I start the four week count down? And surely I can return after to the uk on the tourist 3  month entry period if I have to? How deos one 'end' a work permit?

In addition to what Wishstar has said, you would have received a P60 or P45 giving your dates of employment, and the HO can look it up any time they want.  And they absolutely *will* check it out if you attempt to enter a migration path towards permancy.  That's for sure.

To answer your question specifically, your work permit ends on the date shown on your P60 or P45.  After that you have 28 days.

It is possible to arrange an in-country switch from a WP to a visitor and extend your stay for up to six months and they are pretty empathetic about granting it because it's difficult for a worker to just up and out in 28 days.  *BUT* you need to have unspent leave on your existing visa in order for them to consider it.  If you are without unspent leave when you apply, they can still do an exceptional if you have compelling grounds.  If they make an adverse decision, and you leave the UK with remaining unspent leave, you do not have to record the refusal on subsequent VAF forms.  The tribunal has upheld this point already.

Contrary to what others have said about leaving the UK in the event that you are already illegal:  I would gently remind the folks here that leaving the country is not something you would advise a vandal, burglar, shop-lifter, pickpocket, or other law-breaker to do, is it?   If he's in breach, especially with a documented work history, he needs to get qualified advice and get it sorted out. 

Contrary to what others have said about leaving the UK in the event that you are already illegal:  I would gently remind the folks here that leaving the country is not something you would advise a vandal, burglar, shop-lifter, pickpocket, or other law-breaker to do, is it?   

Nope, wouldn't tell any of them to leave the country.  But then again, none of them have broken immigration law.

If he wants any chance of returning here on a permanent basis, he needs to follow the rules.  Hanging around on an expired work permit is NOT following the rules and will only come back to haunt him if he applies for another type of visa in the future.  He sounds confused and will likely clutch at whatever straws are offered him so that he can stay here with his girlfriend and I think that suggesting that he should stay here on an expired work permit (for whatever reason) isn't sound.  If he wants to sort things out, he can sort them out from the US.

I'd have to agree with Lola on this one.  This situation is entirely different from other law-breaking activities.  Breaking immigration rules are fine...if you don't intend on making a formal application for settlement. 

And I believe I did advise seeking professional advice, in addition to making arrangements to leave if he is in breech of his work visa.

Just a question that I haven't seen answered yet:  what kind of visa is your girlfriend on?  I think we are all assuming that she holds a Work Permit - is this correct?

Oh - I was assuming she was on a student visa?

Good question, saraliz.