If you get Shakespeare's sonnet, then you're well on your way. That poem rejects a lot of the ridiculous, over-the-top comparisons that poets have traditionally used to describe the women they love. The speaker of the poem does not say that his lover isn't beautiful. He simply says that her eyes don't shine like the sun, that her skin is not as white as snow, that her hair is not a mass of fine golden wires, that her voice isn't like music, that she doesn't float through the air like a goddess, etc. Of course, those comparisons wouldn't be true about any woman. A poet who praises his love in such extravagant terms isn't talking about reality. In effect, he's saying that the only way he can love his woman is to turn her into an unreal fantasy object. But the speaker of Shakespeare's sonnet is saying that his woman's beauty and his love for her are so real that they don't need to be gussied up with all that "false compare." For its first 12 lines, the sonnet seems to be putting the woman down and saying that she's not attractive, but the final couplet turns it all around.
Lochhead's poem has very much the same structure. The speaker tells her lover that he could give her this love token and that love token and she wouldn't thank him. For most of the poem, she seems to be saying that she would reject his gifts and thus reject his love, but in the last two words of the poem, she also turns everything around. It turns out that what she has meant all along is that she wouldn't just thank him, she wouldn't merely thank him, she would do much more than thank him. His gifts would do much more than win a simple expression of gratitude. They would melt her heart.