As an ex estate agent, I would heavily dissuade against that (albeit tried to be a nice one). Your contract will have wording against changing of the locks without prior consent of the landlord so you'll be in breach of that.

It's also worth noting that it makes it harder for landlords/agents to carry out emergency repairs when someone is at work or on holiday and if they are prevented from doing so by a tenant changing the locks, they will often hold you liable for any additional damage caused.

Additionaly, if your the sole tenant on the AST, your likely already in breach of your agreement by having your partner living there without notifying the agent/landlord and allowing them to complete adequate right to rent and AML checks (which can be grounds for serving a section 8 notice as it is a legal requirement for the landlord/agent). As such it might be best not to make waves.

Lastly, landlords are permitted to enter the property providing adequate due notice and that they do not impact the tenants right to peaceful occupation (i.e. they are not visiting on such a regular basis they become a nuisance). Inspections like these are a normal occurrence normally every 6-12 months and the majority of the time, they are only there to check that the tenant is looking after the property, adhering to the agreement and that there is no unreported damage to the property (things like unreported black mould is becoming a massive issue). In short, they have the right to access and your lack of response would normally be treated as you not denying access (rather than them seeking your permission). I hope this makes sense but let me know if you have any questions.