Wiring a double oven on a wall opposing the wiring

As long as the ampacity of the existing 220VAC branch circuit is sufficient to support the new cooking equipment, you can do this, despite it not being called out explicitly as allowed for by the NEC. The closest cites I can find in the Code for this are 210.19(A)(3) Exception 1:

Exception No.1: Conductors tapped from a 50-ampere branch circuit supplying electric ranges, wall-mounted electric ovens, and counter-mounted electric cooking units shall have an ampacity of not less than 20 amperes and shall be sufficient for the load to be served. These tap conductors include any conductors that are a part of the leads supplied with the appliance that are smaller than the branch-circuit conductors. The taps shall not be longer than necessary for servicing the appliance.

210.23(C):

40- and 50-Ampere Branch Circuits. A 40- or 50-ampere branch circuit shall be permitted to supply cooking appliances that are fastened in place in any occupancy. In other than dwelling units, such circuits shall be permitted to supply fixed lighting units with heavy-duty lampholders, infrared heating units, or other utilization equipment.

and Table 220.52, Notes 3 and 4 (reformatted to use decimals instead of improper fractions for kilowattage):

3. Over 1.75 kW through 8.75 kW. In lieu of the method provided in Column C, it shall be permissible to add the nameplate ratings of all household cooking appliances rated more than 1.75 kW but not more than 8.75 kW and multiply the sum by the demand factors specified in Column A or Column B for the given number of appliances. Where the rating of cooking appliances falls under both Column A and Column B, the demand factors for each column shall be applied to the appliances for that column, and the results added together.
4. Branch-Circuit Load. It shall be permissible to calculate the branch- circuit load for one range in accordance with Table 220.55. The branch-circuit load for one wall-mounted oven or one counter-mounted cooking unit shall be the nameplate rating of the appliance. The branch-circuit load for a counter- mounted cooking unit and not more than two wall-mounted ovens, all supplied from a single branch circuit and located in the same room, shall be calculated by adding the nameplate rating of the individual appliances and treating this total as equivalent to one range.

Is this normal

No, it is normal to have an electric oven on a separate radial circuit with its own MCB in the consumer unit. For example a dedicated 32A "Cooker" circuit.

Ovens are normally hardwired through a dual-pole isolating switch adjacent to the oven.

and/or safe?

It is safe so long as the installer calculated the loading on the downstairs ring circuit. There are formula to be used which take into account the type of appliances, the number of sockets and the conductor sizes. (see Diversity and Demand below for load calculations with examples)

The REALISTISK Oven has a total rating of 2.7 kW, which is only slightly more than a typical electric kettle. It is also less than 13A on a 230V circuit, so won't be a problem for the actual socket, 13A fused plug and cable. However you obviously can't have all four heating elements on, from cold, at the same time (top, bottom, fan & grill).

it seems to me that it would still be best to isolate the oven from the general plug socket circuit

I believe that would be normal practise.

Useful links

• Radial circuits for socket outlets, cookers, shower and many other items, John Ward. (11:00)
• Maximum Diversity and Demand, John Ward. (16:10)