2-point boundary value analysis is a technique that tests the boundary values of valid and invalid partitions with two values each: one at the boundary and one just outside the boundary. For example, if the valid range is 1 to 10, the 2-point boundary values are 1, 2, 9, 10 for the valid partition, and 0, 11 for the invalid partitions. The advantage of this technique is that it reduces the number of test cases compared to 3-point boundary value analysis, which tests one value inside the boundary as well. However, it may miss some defects that occur only with the values inside the boundary.

In this question, the valid partitions are the ranges of spending that correspond to different discounts, and the invalid partitions are the ranges of spending that are below or above the valid partitions. The boundaries are the values of spending that trigger a change in the discount rate. The 2-point boundary values for each partition are:

Invalid partition below £500: £0, £499

Valid partition for 5% discount: £500, £501

Valid partition for 10% discount: £1000, £1001

Valid partition for 15% discount: £2000, £2001

Valid partition for 20% discount: £4000, £4001

Invalid partition above £4000: £4001, £4002

The test cases TC1 and TC2 already cover two of the boundary values: £500 and £2000. Therefore, to achieve 100% 2-point boundary value analysis, we need to write seven more test cases to cover the remaining boundary values: £0, £499, £501, £1000, £1001, £4000, and £4002. Note that £4001 is a boundary value for both the valid and invalid partitions, so we only need to test it once.

References: ISTQB Certified Tester Foundation Level Syllabus v4.0, Chapter 4.2.2, p. 35-36 1; ISTQB Glossary v4.0, p. 16 2; Software Testing - Boundary Value Analysis 2