I assume that your use of the term MV relates to an ac voltage >1kV, <36kV.
The above link takes you to a relevant section of the Electricity, Safety, Quality & Continuity Regulations which apply to generators, distributors and meter operators in the UK. In these Regs, voltages >1kV are classed as "HV". If your customer belongs to such a category, then operating an unearthed HV circuit would be a breach of these Regs.
Otherwise, provided that you can demonstrate that the system can be operated safely (eg with a Design Risk Assessment), I'm not aware of any reason why this shouldn't be done. I would expect "safe operation" to include a means of interupting the circuit in the event of an earth fault. Such protection would need a VT configured either as a 5-limb 3-phase type, or 3 x 1-phase units supplying a neutral Voltage Displacement protection relay. The VT HV star point must be earthed for satisfactory operation.
The additional cost of the VT may well cause a reconsideration of the step-up transformer to a YNd configuration, with the N earthed (either directly, or via an impedance). This is the approach I'd commend, as this allows "standard" current-operated earth fault protection to be employed on the MV side with either/ or both Balanced EF/ Standby EF. In general, I'd expect to be able to achieve faster fault clearance, whilst avoiding spurious operation, with current-operated EF protection, especially if any of the MV circuit is run as overhead lines.
In any event, I'd recommend that an experienced HV design engineer be employed to sign-off on the final design.