Hi - I’d be grateful for your guidance for our situation.
We signed a 24-month fixed term rental agreement on Aug’19 with expiry Aug’21. We are a family of 4 (with 2 teens boy & girl) and this is a 2 bedroom apartment. But we took it thinking that we will be fine to adjust. We soon realised that its increasingly difficult to manage with less rooms and bathrooms. We started looking for a house and after a long wait during the pandemic, we are now ready for exchange & completion in early July’20. The house needs work, I estimate we will be ready to move only in Oct’20. Hence we will be terminating 10 months early. We haven’t notified the agent yet (thinking of waiting until the completion on our new home). Our rent is £1250 pcm, Kingston,Surrey.
What is the best way to approach this? Are we stuffed??? 

Any guidance in this matter will help us immensely. Thank you.
Here is the clause in the rental agreement, there is no other break clause in the agreement:
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This is a fixed term tenancy agreement with no provision for either party to terminate the agreement before the expiration of the fixed term. Under exceptional circumstances, the Landlord may (but without any obligation), consider a request for the surrender of this tenancy as described below.
If the Tenant requests to surrender the tenancy prior to the end of the fixed term, and the Landlord agrees to the surrender, then the Tenant will be responsible for rent until the earlier of the expiry of the tenancy under this agreement and the date the Landlord or a new tenant takes possession of the Premises. The Tenant will also be responsible for any reasonable costs the Landlord incurs in effecting a surrender of the tenancy and finding a replacement tenant. The Landlord will advise of the actual costs at the time of the request, each case being considered on its own merits. As a general guideline, these costs may include (but not limited to):
• The letting fee of up to 11% + VAT (13.2%) of the rent from the date of early surrender until the original end date of the Tenancy,
• A proportion (being the number of months to be surrendered early as a percentage of the current fixed term) of the Landlord’s costs in sum of £144 (inc.) for an inventory clerk checking the new Tenant into the Property,
• A proportion (being the number of months to be surrendered early as a percentage of the current fixed term) of the Landlord’s costs in sum of £480 (inc.) being the cost of administering the new Tenancy, preparing the Tenancy Agreement, collating references, checking the new Tenant’s Right to Rent and registering the deposit with an approved Government scheme
• For the surrender and grant of a new tenancy on similar terms, the Landlord’s costs would be £300 (inc.) plus any associated inventory costs.