OK. Clarification.
UK - The qualifying years for a full pension changed in April, 2010 and now stands in transition to 30, depending on your date of birth. So, by the end, if you worked in the UK, have 15 qualifying years, you will receive 50% of the basic State pension (15/30, and provided you paid the minimal National Insurance contribution). The UK State pension is taxable by the UK.
US - The minimal qualifying years for Social Security benefits is 10 years (40 credits), NORMALLY. But…..
The US has a special Totalization Agreement with the UK. This allows you to use UK State pension qualified years to help you qualify for the US Social Security benefits. The UK years do not leave the UK, and still count towards the UK State Pension (i.e.: 15 years above are still worth 15 years).
You have 6 years (24 credits) in the US. The SSA (Social Security Administration) would use these, and add 4 from your UK qualified years to allow you to meet the minimum US requirement. Again, the UK years do not leave the UK. They still count towards the UK State Pension. You now qualify for US Social Security benefits. The actual benefit calculations would only be made on the 6 years worth of contributions, but as they say, better than a sharp stick in the eye. The UK Pension Service must do this for you, and you must tell them that you have US contributions when applying for the UK State Pension. The Social Security benefits are not taxable in the US (by the treaty), but are taxable in the UK. Your payments are made from the US, which requires you to fill in a UK self assessment tax form (no big deal), since no UK tax is deducted, but for which you are liable. You paid money into the US SSA system, so ....
There are even more benefits (and good ones, if you’re married) once you qualify for the SSA benefits, but you'll discover them on your own. I don't wish to bankrupt US Social Security any sooner than necessary.
This is all according to pre2010 UK pension rules. I've seen nothing that changes this as of yet.
Footnote:
I see from the exchanges that you have time in Germany. There is a similar agreement to the US/UK agreement between Germany and the UK (EEA agreement). It works much the same, and retirement benefits from the Continent are similar in value to the US. Again, be sure to alert the UK Pension Service that you have these contributions and to access these funds due you by qualifying you for the German system.
My reason for bringing all this up is that you seem to be under the impression that your US benefit years can be used to help your UK State pension. Under the UK rules, it appears that although there is discussion of calculating them as additional years, the actual amount of your UK pension will NOT increase above the number of qualifying years you have in the UK at the time of retirement. In most countries, including the UK, the input of contributions from abroad are only used to qualify you for the pension, and not to “top the pension up”.
This is why it is important to be sure to claim the US and German pensions. I imagine the view of the UK Pension Service is that you are entitled to the US and German pensions, so there is no reason for the UK to pay for them through additional UK benefits. (You claim from each system based on what you contributed to that system.)