How can I thank a senior faculty who's written me a good reference which helped me land a desirable job?
A thank you card is definitely a good idea and I cannot see that it would be considered inappropriate. If you are from abroad, a little something typical of your country of origin may also be appropriate. Like a small box of traditional sweets, a little traditional doll, a fan, or something like that. I have had this kind of little things given to me by students.
Also, the university will probably have a policy on gifts which you may be able to locate on their web site. The general idea is that it should not be something worth a significant amount of money.
Rather than asking a classmate, I would ask a junior faculty for what goes as the norm. A classmate may be as inexperienced in this as you are, because the rules in academia can be significantly different from other parts of society.
A thank-you card would be perfect, or even just saying thank you.
There isn't a strong culture of giving thank-you gifts in the UK so your professor won't expect one and certainly won't feel that you are ungrateful if you don't give him something. However, it's almost never inappropriate1 to give a small gift if that's what you want to do. The suggestions in the other answers (a gift card for a coffee shop, a bottle of wine or especially something from your own country) are all good ideas. I wouldn't spend more than about £10 on such a gift.
1 The situations where it would be inappropriate would mostly be cases where the gift could be interpreted as a bribe; that obviously isn't the case, here.
A well thought-out thank you note would be good. Better if you know them well enough to know they like a particular place, e.g. their favorite coffee shop, and you could buy them a gift certificate. I'm probably reading my own biases into this, but I would not get them less than a $20 gift certificate, or maybe a nice bottle of wine (if you know they'd like it). If that's too expensive for you, just focus on making your thank you note mean more than a couple cups of free coffee.
But you're under no obligation. They performed their duty as they were obliged to do, likely because you did so as well. If you're on good terms, and I think it's safe to assume you are, a simple thank-you note is likely to mean much more than anything else you might do.