Yes, you can ask for anything, but by and large landlords would rather keep to 6m renewable ASTs. The reason for this is that if you have a long lease and default on rent it is much harder to evict you. Most landlords with defaulting tenants just wait until the end of the 6m, where eviction is legally much easier. This is an example of how successive governments have moved the law in favour of the tenant, but in the end it backfires on you. I should say that landlords are in the business of rent from clean and quiet tenants, and will normally renew ASTs automatically.