Is the mortgage from ‘the Islamic Bank of Britain’ allowed in Shariah?

assalamualaykum, i am currently a lease holding tenant for a business. i am thinking to buy this business as a free hold, however i will have to get a mortgage from ‘the islamic bank of britain’ they say that this ‘islamic type’ mortgae is allowed in shariah. they charge rent instead of intrest but they charge this rent according to the goverment intrest rate. e.g. if i put 25% of capital, 75% is the banks share. as i repay my debt my share of the property increases and the banks share decreases. The bank also shares insurance premiem & building maintanance cost according to percentage share. please tell me is this type of mortage, from ‘the islamic bank of britain’ is allowed in shariah. please reply to this question. jazakallah