The banks and building societies aren't being as generous as they try to let on, 3 months payments on a 25 year mortgage is 1% of the total amount you're going to pay. Depending on how much the borrower has left on the term, they can recoup that money by just adding a few pounds a month on the payments plus they still have the ultimate get out of jail card that the mortgage is secured on the house, they're not actually risking much and it saves them the expense both in money and public image of mass evictions.
Renting is a totally different kettle of fish, if the landlord lets the tenant off three months rent then the landlord is entirely bearing the risk, there is nothing he can do except evict them from non-payment either via section 8 which is expensive or section 21 which takes time (and thus more rent arrears). Either way he ends up with thousands in debt and pretty much no hope of ever getting it back.
No landlord is going to agree to rent holidays unless he is absolutely sure that the tenant is going to repay at some future date or someone (which means the Govt) is willing to step up and underwrite the risk. Is the current Govt willing to do that? I doubt it The current bunch are a bit more socially concious than Cameron/Osbourne and their "Let Them Eat Cake" approach but I am doutbful they would go so far as agreeing to underwriting unpaid rents.