I want to start practising for interviews as will be looking to move next year but do not want to leave my current role which is paying me well.
My fear is that if I interview with a company just for the sake of practising and if I receive the offer, declining it will feel manipulative as I never intended to work there.
How do I deal with this?
No, I don't think it's manipulative.
But I think it's important to communicate and set expectations appropriately. Consider two ways of talking to a hiring manager:
If you say #1 but are actually just using the company for interview practice, that could sour a relationship.
If you say #2, a good founder/hiring manager will have the attitude of "we are going to impress this candidate and they'll have a hard time turning down our offer." Give the company a chance to showcase themselves as you meet their team.
I don't think it's manipulative, because you can't predict how their offer will turn out. There is always that 1% chance they could surprise you and you end up going there.
As a personal anecdote, a friend of mine interviewed with 2 no-name companies and thought nothing of it. He got offers from both, and both of them were pretty weak (given that they were no-name companies). However, he negotiated extremely well and pitted those 2 offers against each other, going through 3 rounds of negotiation. By the end, he tacked on +$50k to both of the offers and accepted one of them. He gave his all to that company and worked there ~3 years.
If you get an interview opportunity, even if you're not likely to go there, you should take it, especially in this market. Treat people in the loop as best you can and best of luck!