Item held by customs until I pay tax; what are my options?

I just arrived in the UK yesterday, from the US, carrying a piece of jewelry, which I intend to carry with me for the following months before I give it to someone at the right time and place, then presenting it to a special someone.

Immigration discovered the jewelry while searching my bags for unrelated reasons^*, and informed me that I should discuss the jewelry with customs later to see if a duty needed to be paid, which I did.

The relevant bit is, because the intended recipient is an EU citizen (with dual Spanish/Guatemalan citizenships), currently living in the UK, even though I will not be giving the gift to her in the UK (or in the EU--I intend to gift the item in January in Guatemala, after she has moved back home to her family), the customs agent determined that I must pay a tax of £510 before I can retrieve my item.

As I did not have that much money available when I arrived, I said I would return Friday to pay the tax and collect the item.

Aside from trying again, most likely in vain, to explain that I will retain the item for the duration of my stay, do I have any possible recourse? Can I ship the item out of the EU rather than collecting it in the airport? Or could I board a flight leaving the EU with the item, rather than paying the tax? Or anything else?

^*While I believe the details of the search are irrelevant to this question, it's been asked about many times, and it's reasonable that it might be relevant under some circumstances, so I will elaborate:

• I arrived without a booked ticket for onward travel, which raised suspicion
• I had only carry-on baggage, which further raised suspicion
• I arrived with minimal cash, and primarily debit/credit cards
• After some questions about my work, financial means, etc (all routine), the immigration agent searched my bags
• When he came across the ring, I explained its purpose, and he called over a coworker, and they agreed that I should discuss it with customs, even though I wasn't planning to leave it in the UK
• After the immigration agent was done with me, he simply showed me to the customs area, and left me. He made no effort to follow me, or hand me over directly to customs, so he didn't appear to have any worries that I would try to evade customs.

I have entered the UK multiple times under identical circumstances (most recently less than a month prior), and have never gone through a search, so I didn't think anything of it. The particular agent, however, was training a new recruit, may have made him examine everyone more closely for training purposes, but for whatever reason, as luck would have it, he chose to search my bags, although that has never happened before to me.