Even if you had entered using your passport, you would have no passport stamps, because EU countries do not stamp the passports of EU citizens. The problem of having no passport stamps is therefore one that applies to every EU, EEA, or Swiss citizen who wants to prove presence in the UK.
You will first need to apply to have the UK recognize your status as a permanent resident. This is outlined on the relevant gov.uk page on becoming a British citizen:
There are different ways to become a British citizen. The most common is called ‘naturalisation’.
You can apply for British citizenship by naturalisation if:
- you’re 18 or over
- you’re of good character, for example, you don’t have a serious or recent criminal record, and you haven’t tried to deceive the Home Office or been involved in immigration offences in the last 10 years
- you’ll continue to live in the UK
- you’ve met the knowledge of English and life in the UK requirements
- you meet the residency requirement
And you must usually have:
- lived in the UK for at least the 5 years before the date of your application
- spent no more than 450 days outside the UK during those 5 years
- spent no more than 90 days outside the UK in the last 12 months
- had settlement (‘indefinite leave to remain’) in the UK for the last 12 months if you’re from outside the European Economic Area (EEA)
- had permanent residence status for the last 12 months if you’re a citizen of an EEA country - you need to provide a permanent residence document
- not broken any immigration laws while in the UK
(Emphasis added.) The item in bold means that you'll need to get a permanent residence document and then wait at least a year before you can apply for naturalization.
There are different requirements if your spouse or civil partner is a British citizen.
That bit links to the next page, which seems to say that once you marry your girlfriend, the 12-month waiting period no longer applies, but you'll still need the permanent residence card:
If you’re married to, or the civil partner of, a British citizen, you can apply for citizenship if:
- you’re 18 or over
- you’re of sound mind, you’re able to think and make decisions for yourself
- you’re of good character, for example you don’t have a serious or recent criminal record
- you’ve met the knowledge of English and life in the UK requirements
- you’ve been granted indefinite leave to stay in the UK (this means there’s no specific date that you have to leave) or permanent residence if you’re an EEA national (and you have a permanent residence card or document that shows you have permanent residence)
- you meet the residency requirement
Unless your spouse or civil partner works abroad either for the UK government or for an organisation closely linked to government, you must usually also have:
- lived in the UK for at least the 3 years before your application is received
- spent no more than 270 days outside the UK in those 3 years
- spent no more than 90 days outside the UK in the last 12 months
- not broken any immigration laws while in the UK
The page on getting a permanent residence card confirms this:
You can only use your permanent residence document to apply for British citizenship after you’ve lived in the UK for 6 years.
That means you must wait another 12 months if you’ve only lived in the UK for 5 years when you get your document.
But you can apply immediately if:
- you’ve already lived in the UK for 6 years when you get your document
- your husband, wife or civil partner is a British citizen
I'm afraid your first year of study in the UK doesn't count, since you left for a year. That lengthy absence, I believe, means that your 5-year period of qualifying residence started after you returned, which means that for the purposes of acquiring permanent residence and qualifying to apply for naturalization, you have lived in the UK for only 5 years. I am not completely certain about this, though, so you may want to look at that question more closely.
Finally, to answer the question you posted, the page on getting a permanent residence card mentions some types of evidence you'll need and examples of documents you can use:
- evidence that you’ve been living in the UK, such as gas, electricity and council tax bills, and letters from government departments
- evidence that you’ve been working, studying, self-employed, self-sufficient or looking for work, such as payslips, P60 forms and bank statements
The page also notes that
You’ll need to send other documents depending on your situation - check the form and guidance.
You may be able to apply online, or you may need to submit a paper application form. This depends on whether you, as a student, are reliant on someone else for financial support, or are providing financial support to someone else:
You can apply online for a permanent residence document if you’re from the EEA.
You can’t use this service if you’re a student or self-sufficient person and you’re either:
- reliant on a family member for financial support
- financially responsible for any other family members
Download and complete form EEA (PR) if you can’t apply online. Read the guidance before you fill in the form.
Post it to the Home Office address on the form, with the £65 fee and supporting documents listed.
The "download and complete" link above links to a page from which you can download both the form and the guidance. Do not overlook the advice to read the guidance before you fill in the form.
