The answer is likely to be yes. Under the Equality Act 2010 the GP practice will discriminate against the patient if they treat him unfavourably because of something arising in consequence of the patient's disability. If the patient wasn't deaf, he would be entitled and able to access counselling services if offered to him. It is not proportionate to not offer counselling because of the cost to provide an interpreter. As the practice is providing a public service, it has a duty to make reasonable adjustments to ensure that it is not discriminating against the patient. On the information provided, £100 per session for 6 sessions is likely to be considered a reasonable adjustment to make.
The practice must also consider the public sector equality duty. This duty applies to them as they are carrying out a public function. This requires, amongst other things, to have due regard to the need to eliminate disability discrimination. To not provide a service because of a patient's disability would work against the purpose of the duty.