The Maximum electric or nominal power of your PV system can be defined as its 'Peak Power' (in Watt Peak). The Peak Power of solar panels is registered under the following Standard Test Conditions: a light intensity of 1.000 W/m² sunlight hitting the positioned solar cells perpendicularly. In a laboratory at perfect ambient temperature with the sunlight at 90 degrees to the panel this is what you can expect to generate.
In reality you will only get close to that on clear sunny cool days in the summer but not on an average day in Scotland or anywhere else in the UK. Especially in the dark days of winter.
We use a rough guestimate in the UK of 80% peak summer midday, South facing. Meaning a 400W panel would be estimated to produce a peak wattage of around 320W in UK summer. This can drop by up to 90% in peak Winter, especially if poorly angled eg Flat.