To quote from the UK AIP ENR 1.6.2. - SSR OPERATING PROCEDURES:
1. General
1.3 With the exceptions detailed in paragraph 2 pilots shall:
(d) Select Mode C simultaneously with Mode A unless otherwise instructed by an ATS unit;
2.2 Conspicuity Code
2.2.1 When operating at and above FL 100 pilots shall select code 7000 and Mode C except:
(a) When receiving a service from an ATS Unit or Air Defence Unit which requires a different setting;
(b) when circumstances require one of the Special Purpose Codes or one of the other specific conspicuity codes assigned in accordance with the UK SSR Code Allotment Plan as detailed in the table at ENR 1-6-2-3 to ENR 1-6-2-6.
2.2.2 When operating below FL 100 pilots should select code 7000 and Mode C except as above.
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The only time that the transponder should be selected without Mode C is when instructed by ATC or when the Mode C is u/s. Any instructor who tells you anything different is wrong and is probably not the best person to learn from. Consider changing to an instructor who knows his job.