If all the contracts are with other businesses (ie B2B) then you are comparatively free to define the termination rights as you choose, although they should of course be set out clearly in the T&Cs.
If you are entering into contracts with consumers (B2C) then you are not so free. Most notably, you need to allow for cancellations under the distance selling laws, currently contained in the Consumer Contracts (Information, Cancellation and Additional Charges) Regulations 2013. You cannot opt-out of these. Also, you need to consider the effects of the Consumer Rights Act 2015. "Unfair terms" under that legislation may include:
"A term which has the object or effect of authorising the trader to dissolve the contract on a discretionary basis where the same facility is not granted to the consumer, or permitting the trader to retain the sums paid for services not yet supplied by the trader where it is the trader who dissolves the contract."
"A term which has the object or effect of automatically extending a contract of fixed duration where the consumer does not indicate otherwise, when the deadline fixed for the consumer to express a desire not to extend the contract is unreasonably early."
See Schedule 2 to the 2015 Act.