I got insurance last month from Direct Line. I live in a rented property too and they haven't actually asked what I'm insuring. Contents cover up to £50k, with valuables (jewelery etc) not exceeding £15k of that amount. Guitars and stuff are covered, but not counted as valuables. £3.5k of personal cover for when me and the missus are out and about (her engagement and wedding rings and my watch).
I think you only have to declare any item worth over £2k. Costs me about £15 a month, but they had an offer on at the time.